chore(Rakefile): parallelize the build on Travis

now that the forking issue is solved we can run regular build there

https://github.com/travis-ci/travis-ci/issues/845

Rakefile

@@ -105,11 +105,7 @@ task :minify => [:init, :concat, :concat_scenario] do
     'angular-bootstrap.js',
     'angular-bootstrap-prettify.js'
   ].each do |file|
-    unless ENV['TRAVIS']
-      fork { closure_compile(file) }
-    else
-      closure_compile(file)
-    end
+    fork { closure_compile(file) }
   end
 
   Process.waitall