Use Bundler to make it easier for contributors to compile Sass

It's common for Sass projects to include a Gemfile that lists all the gem dependancies of the project. This can reduce the potential for errors because of missing/incorrect versions of gems. It also reduces complexity for contributors because all the need to do is run 'bundle install' to load all the required gems for the project.

Here is an example of a Gemfile from Omega 4.x

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