Prevent syntax errors on Ruby 1.8.7

Version 1.6.0 added some Ruby 1.9 hash syntax, which obviously gives 1.8.7 a fit. Prior to 1.6.0, my test suite passes on with Ruby 1.8.7, and it also does with this commit. 

I wasn't able to get your test suite running with 1.8.7 due to some of the development gems, but from what I can tell Parslet works with 1.8.7 just fine.

Great work on Parslet by the way. :thumbsup: