(fix) changed to chain the process() promises

The previous implementation ran process() parallel on one processor instance
which messed up variables when plugins auto generated fallback styles for
CSS variables for example.

tasks/postcss.js

@@ -123,6 +123,8 @@ module.exports = function(grunt) {
 
             tally.diffs += 1;
         }
+
+        return Promise.resolve();
     }
 
     grunt.registerMultiTask('postcss', 'Process CSS files.', function() {
@@ -149,9 +151,9 @@ module.exports = function(grunt) {
         }
 
         var done = this.async();
-        var tasks = [];
+        var taskChain;
 
-        this.files.forEach(function(f) {
+        taskChain = this.files.reduce(function(previousFileset, f) {
             var src = f.src.filter(function(filepath) {
                 if (!grunt.file.exists(filepath)) {
                     grunt.log.warn('Source file ' + chalk.cyan(filepath) + ' not found.');
@@ -165,24 +167,28 @@ module.exports = function(grunt) {
             if (src.length === 0) {
                 grunt.log.error('No source files were found.');
 
-                return done();
+                // Complete this file set, don't fail all file sets.
+                return Promise.resolve();
 
             // This check shall not apply for configs without destination
             // (in place processsing).
             } else if (src.length > 1 && f.dest) {
                 grunt.log.warn("Multiple source files with one destination file configured. All files would write to that *one* destination file.\nThe configured file set is: " + f.src.join(", ") + " => " + f.dest + ".");
             }
 
-            Array.prototype.push.apply(tasks, src.map(function(filepath) {
-                var dest = f.dest || filepath;
-                var input = grunt.file.read(filepath);
+            return previousFileset.then(function() {
+                return src.reduce(function(previousTask, filepath) {
+                    var dest = f.dest || filepath;
+                    var input = grunt.file.read(filepath);
 
-                return process(input, filepath, dest)
-                    .then(writeResult.bind(null, tally, input, dest));
-            }));
-        });
+                    return previousTask
+                        .then(process.bind(null, input, filepath, dest))
+                        .then(writeResult.bind(null, tally, input, dest));
+                }, Promise.resolve() /* to kick-off the chain */ );
+            });
+        }, Promise.resolve() /* to kick-off the chain */ );
 
-        Promise.all(tasks).then(function() {
+        taskChain.then(function() {
             if (tally.sheets) {
                 if (options.writeDest) {
                     grunt.log.ok(tally.sheets + ' processed ' + grunt.util.pluralize(tally.sheets, 'stylesheet/stylesheets') + ' created.');