Add ruby versions this gem works
RSpec's bind is defined in twice

When executing rspec, the following message appears:

> Your Gemfile lists the gem rspec (>= 0) more than once.
> You should probably keep only one of them.
> While it's not a problem now, it could cause errors if you change the
> version of just one of them later.

`gem 'rspec'` is defined in the both Gemfile and .gemfile.
so removed it from Gemfile.
Merge pull request #4 from satoryu/master

This gem should support ruby 1.9.3 or later