chore(Rakefile): skip build parallelization on Travis

Due to a infrastructure change on Travis starting JVMs in forked
processes doesn't currently work. Since we don't really care that
much about the build speed on Travis, I'm going to disable it there.

Related issue: travis-ci/travis-ci#845

Rakefile

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