Fixed parser name. Fixed Gemfile and Rakefile.

Hi @0xfe 
I made some minor changes to your library. First of all I fixed an issue with the naming of the parser exported by vextab.coffee, that was causing the `vextab_parser is undefined` as mentioned in issue #49 and also was failing all the tests.  
I also switched the `sh` command in the Rakefile, migrating to the backquote command. This was causing an issue when running the rake tasks in a ZSH shell, using rbenv (the wrong ruby was called).  
I removed therubyracer from the Gemfile, and added libv8 instead. therubyracer has some issues right now with OsX Yosemite.  

Thanks for your great work.  
Cheers