chore(travis): speed up the build

- parallelize the tasks
- cache requests (e2e tests)

This reduces the time from ~18min to ~12min.

It makes the output little messy. We could buffer output of each task and display it once it's fully finished, nicely. I think giving instant feedback is better.

lib/grunt/utils.js

@@ -36,11 +36,13 @@ module.exports = {
     var browsers = grunt.option('browsers');
     var reporters = grunt.option('reporters');
     var noColor = grunt.option('no-colors');
+    var port = grunt.option('port');
     var p = spawn('node', ['node_modules/karma/bin/karma', 'start', config,
       singleRun ? '--single-run=true' : '',
       reporters ? '--reporters=' + reporters : '',
       browsers ? '--browsers=' + browsers : '',
-      noColor ? '--no-colors' : ''
+      noColor ? '--no-colors' : '',
+      port ? '--port=' + port : ''
     ]);
     p.stdout.pipe(process.stdout);
     p.stderr.pipe(process.stderr);
@@ -235,5 +237,24 @@ module.exports = {
       }
       next();
     };
-  }
+  },
+
+  parallelTask: function(name) {
+    var args = [name, '--port=' + this.lastParallelTaskPort];
+
+    if (grunt.option('browsers')) {
+      args.push('--browsers=' + grunt.option('browsers'));
+    }
+
+    if (grunt.option('reporters')) {
+      args.push('--reporters=' + grunt.option('reporters'));
+    }
+
+    this.lastParallelTaskPort++;
+
+
+    return {grunt: true, args: args};
+  },
+
+  lastParallelTaskPort: 9876
 };