Change Ruby version from 1.9.2 to 1.9.3

Support for Ruby 1.9.2 was removed from on Sep 17, 2013:
https://github.com/guard/guard/commit/17f32f8000f8e02e6ad4c3a46f7125582b2f6c4c
Merge pull request #550 from kiela/master

Change Ruby version from 1.9.2 to 1.9.3