Add ability for user to include therubyracer gem

Prompt the user to include 'therubyracer' gem when using the
application template.  If user answers 'yes' to prompt, add the
following to the Gemfile and commit the change:

gem 'therubyracer', platforms: :ruby

HYDRASIR-77 #close
Merge pull request #398 from projecthydra/feature/HYDRASIR-77-add-rubyracer-option-application-template

Add ability for user to include therubyracer gem