Added less-rails-bootstrap dependency

I just tried installing shopify_app on a clean Rails app & RVM gemset. When I ran the generator I got this error:

    /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/resolver.rb:287:in `resolve': Could not find gem 'less-rails-bootstrap (>= 0) ruby' in the gems available on this machine. (Bundler::GemNotFound)
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/resolver.rb:161:in `start'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/resolver.rb:128:in `block in resolve'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/resolver.rb:127:in `catch'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/resolver.rb:127:in `resolve'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/definition.rb:178:in `resolve'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/definition.rb:113:in `specs'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/definition.rb:158:in `specs_for'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/definition.rb:147:in `requested_specs'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/environment.rb:23:in `requested_specs'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/runtime.rb:11:in `setup'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler.rb:116:in `setup'
    	from /Users/craig/.rvm/gems/ruby-1.9.2-p0@global/gems/bundler-1.2.2/lib/bundler/setup.rb:17:in `<top (required)>'
    	from <internal:lib/rubygems/custom_require>:29:in `require'
    	from <internal:lib/rubygems/custom_require>:29:in `require'

This PR adds `less-rails-bootstrap` as a gemspec dependency.