fix a syntax error in Gemfile

this happened for me on macosx + homebrew ruby1.9 + latest bundler.

```
$ bundle install
/Users/sridharr/as/s/discourse/Gemfile:35:in `evaluate': compile error
/Users/sridharr/as/s/discourse/Gemfile:35: syntax error, unexpected ':', expecting $end
...imple_handlebars_rails', path: 'vendor/gems/simple_handlebar...
                              ^ (SyntaxError)
       /Library/Ruby/Gems/1.8/gems/bundler-1.1.5/lib/bundler/definition.rb:18:in `build'
       /Library/Ruby/Gems/1.8/gems/bundler-1.1.5/lib/bundler.rb:135:in `definition'
       /Library/Ruby/Gems/1.8/gems/bundler-1.1.5/lib/bundler/cli.rb:220:in `install'
       /Library/Ruby/Gems/1.8/gems/bundler-1.1.5/lib/bundler/vendor/thor/task.rb:22:in `send'
       /Library/Ruby/Gems/1.8/gems/bundler-1.1.5/lib/bundler/vendor/thor/task.rb:22:in `run'
       /Library/Ruby/Gems/1.8/gems/bundler-1.1.5/lib/bundler/vendor/thor/invocation.rb:118:in `invoke_task'
       /Library/Ruby/Gems/1.8/gems/bundler-1.1.5/lib/bundler/vendor/thor.rb:263:in `dispatch'
       /Library/Ruby/Gems/1.8/gems/bundler-1.1.5/lib/bundler/vendor/thor/base.rb:386:in `start'
       /Library/Ruby/Gems/1.8/gems/bundler-1.1.5/bin/bundle:13
       /usr/bin/bundle:19:in `load'
       /usr/bin/bundle:19
There was an error in your Gemfile, and Bundler cannot continue.
```