Adds a ruby-version file to keep us locked to < v2.0

@searls from our conversation on https://github.com/searls/jasmine-rails/commit/839534213fd5cb791b10f5f428d523abbddfbd87#commitcomment-4481237

Rails 3.1 doesn't play nicely with Ruby v2.0 and throws some weird
errors.

```
"/Users/zmoazeni/projects/jasmine-rails/gemfiles/rails-3.1"
Running `phantomjs
"/Users/zmoazeni/projects/jasmine-rails/lib/jasmine_rails/../assets/javascripts/jasmine-runner.js"
"file:///Users/zmoazeni/projects/jasmine-rails/spec/testbeds/rails-3.1/spec/tmp/runner.html?spec="`
Running:
file:///Users/zmoazeni/projects/jasmine-rails/spec/testbeds/rails-3.1/spec/tmp/runner.html?spec=
Starting...
baz : returns the first thing
  ReferenceError: Can't find variable: baz in
file:///Users/zmoazeni/projects/jasmine-rails/spec/testbeds/rails-3.1/spec/tmp/assets/jasmine-specs.js
(line 878)
baz: 0 of 1 passed.
foo : then $('body').length === 1
  Then clause  `$('body').length === 1` failed by throwing:
ReferenceError: Can't find variable: $
foo : then _([1, 2, 3]).first() === 1
  Then clause  `_([1, 2, 3]).first() === 1` failed by throwing:
ReferenceError: Can't find variable: _
foo: 0 of 2 passed.
bar : then bar.hi() !== "bye"
  Then clause  `bar.hi() !== "bye"` failed by throwing: ReferenceError:
Can't find variable: bar
bar: 0 of 1 passed.
```