Grunt + Postcss + autoprefixer not adding vendor prefixes - gruntjs

Question

Something is definitely wrong somewhere. My test file 'dist/css/prefixedpostcss-main.min.css' is well created but no vendor prefix are added :(
Thanks for advance,
spending lot of time on searching, reading… need the help of the community!
Here is below my Gruntfile.js
module.exports = function(grunt) {
// Project configuration.
grunt.initConfig({
pkg: grunt.file.readJSON('package.json'),
/* grab the Bootstrap js and combine it with my custom scripts
The goal is to limit the number of http requests to increase load time
*/
// JS Concatenation Plugin
concat: {
options: {
stripBanners: false,
sourceMap: true,
banner: '',
},
scripts: {
src: ['bower_components/bootstrap-sass/assets/javascripts/bootstrap.min.js', 'js/plugins.js', 'js/main.js'],
dest: 'dist/js/scripts.js',
},
},
// JS Minification
uglify: {
defer: {
src: ['js/defer.js'], //input
dest: 'dist/js/defer.min.js' //output
},
scripts: {
src: ['dist/js/scripts.js'], //input
dest: 'dist/js/scripts.min.js' //output
},
},
// Sass to CSS
sass: {
dist: {
files: {
'dist/css/main.css': 'sass/globals.scss' // this is our main scss file
}
},
options: {
compass: true,
style: 'compressed',
sourceMap: true
}
},
// Post CSS (autoprefixer, mqpacker, cssnano)
postcss: {
options: {
map: true,
// Load plugins
processors: [
require('autoprefixer')({browsers: ['last 1 version']}),// Runs Post CSS Autoprefixer
//require('postcss-flexbugs-fixes'),
]
},
dist: {
src: 'dist/css/main.css',
dest: 'dist/css/prefixedpostcss-main.min.css'
}
},
/* Auto Update the scripts and styles when working */
watch: {
grunt: { files: ['Gruntfile.js'] },
js: {
files: ['js/*.js'],
tasks: ['jshint', 'concat', 'uglify'],
options: {
spawn: false,
},
},
sass: {
files: ['sass/**/*.scss'],
tasks: ['sass', 'postcss'],
options: {
spawn: false,
},
},
options: {
livereload: false,
spawn: false
}
},
browserSync: {
dev: {
bsFiles: {
src: [
'sass/*.scss',
'js/*.js',
'*.html',
'templates/*.html'
]
},
options: {
watchTask: true,
server: './'
}
}
},
htmlbuild: {
dist: {
src: 'src/index.html',
dest: 'dist/',
options: {
beautify: true,
relative: true,
basePath: false,
sections: {
layout: {
header: 'templates/header.html',
footer: 'templates/footer.html'
}
},
}
}
},
imagemin: {
dynamic: {
files: [{
expand: true,
cwd: 'img/',
src: ['**/*.{png,jpg,gif}'],
dest: 'dist/img/build/'
}]
}
},
jshint: {
files: {
src: ['*.js', 'js/*.js']
},
options: {
'curly': true,
'eqeqeq': true,
'eqnull': true,
'immed': true,
'noarg': true,
'quotmark': 'single',
'trailing': true,
'undef': true,
'unused': false,
'node': true,
'jquery': true,
'browser': true,
'devel': true,
}
},
});
// END GRUNT
// Load the plugins
grunt.loadNpmTasks('grunt-contrib-concat');
grunt.loadNpmTasks('grunt-contrib-uglify');
grunt.loadNpmTasks('grunt-contrib-sass');
grunt.loadNpmTasks('grunt-contrib-imagemin');
grunt.loadNpmTasks('grunt-contrib-jshint');
grunt.loadNpmTasks('grunt-sass');
grunt.loadNpmTasks('grunt-postcss');
grunt.loadNpmTasks('grunt-contrib-watch');
grunt.loadNpmTasks('grunt-browser-sync');
grunt.loadNpmTasks('grunt-html-build');
grunt.loadNpmTasks('grunt-notify');
// Default task(s).
//grunt.registerTask('dev', ['browserSync', 'watch']);
//grunt.registerTask('css', ['sass', 'postcss', 'autoprefixer', 'imagein']);
//grunt.registerTask('js', ['jshint', 'concat', 'uglify']);
//grunt.registerTask('default', ['dev', 'css', 'js', 'notify']);
grunt.registerTask('default', ['sass', 'postcss:dist', 'concat', 'uglify', 'browserSync', 'watch']);
};
Here is below my package.json
{
"name": "laurene",
"version": "1.0.0",
"dependencies": {
"grunt-cssnano": "^2.1.0",
"pixrem": "^3.0.2"
},
"devDependencies": {
"autoprefixer": "^6.7.7",
"grunt": "^1.0.3",
"grunt-browser-sync": "^2.2.0",
"grunt-contrib-concat": "^1.0.1",
"grunt-contrib-imagemin": "^3.1.0",
"grunt-contrib-jshint": "^2.0.0",
"grunt-contrib-sass": "^1.0.0",
"grunt-contrib-uglify": "^1.0.1",
"grunt-contrib-watch": "^1.0.0",
"grunt-cssnano": "^2.1.0",
"grunt-html-build": "^0.7.1",
"grunt-notify": "^0.4.5",
"grunt-postcss": "^0.8.0",
"grunt-sass": "^1.2.0",
"grunt-spritesmith": "^6.6.2",
"gulp-autoprefixer": "^6.0.0",
"postcss-cli": "^2.5.2",
"postcss-flexbugs-fixes": "^2.0.0"
}
}