gem install only on ruby 2.0+

This gem is supported only ruby 2.0+, but we can `gem install` on ruby 1.9.

So I think I should not `gem install` less than ruby 2.0

## example
```bash
$ ruby -v
ruby 1.9.3p545 (2014-02-24 revision 45159) [x86_64-darwin13.1.0]

$ gem install --local pkg/proc_to_ast-0.0.6.gem
ERROR:  Error installing pkg/proc_to_ast-0.0.6.gem:
	proc_to_ast requires Ruby version >= 2.0.0.
```