Replace :rubygems in Gemfile with 'https://rubygems.org'.

Bundler some time ago started display warning message like follow:

The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or
'http://rubygems.org' if not.

So, this change is to remove it and let everything go smother.
Add 'parallel:rake' rake task to run rake tasks in parallel.

This is a transformation of original idea of 'parallel:run_tasks'. See
detailed discussion in pull request
https://github.com/grosser/parallel_tests/pull/161
docs for parallel:rake
v0.9.4
Merge pull request #183 from zekefast/fix_bundle_warning_about_insecure_source_in_gemfile

Replace :rubygems in Gemfile with 'https://rubygems.org'.