Remove rubyforge_project from gemspec

RubyForge does not exist
Simplify files list in gemspec

Follow the convention of other rails projects, which is to include only
the files needed to run the gem. This includes everything in lib.

There is no need to distribute the test files or development files
(Rakefile, Gemfile, .gitignore, .travis.yml, etc.).
Simplify gem build

* Remove sass-rails.gemspec.erb
* There are no executables

To build the gem, just run `rake build`
Merge pull request #277 from teeparham/gemspec

Cleanup gemspec

Conflicts:
	sass-rails.gemspec
	sass-rails.gemspec.erb