Fix gemspec

This fixes a rubygems warning when installing the gem

```
country_select at /Users/scudco/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/bundler/gems/country_select-d261dc86381d did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  [".gitignore
.rspec
.travis.yml
Appraisals
Gemfile
MIT-LICENSE
README.md
Rakefile
VERSION
country_select.gemspec
gemfiles/actionpack3.0.gemfile
gemfiles/actionpack3.0.gemfile.lock
gemfiles/actionpack3.1.gemfile
gemfiles/actionpack3.1.gemfile.lock
gemfiles/actionpack3.2.gemfile
gemfiles/actionpack3.2.gemfile.lock
gemfiles/actionpack4.0.gemfile
gemfiles/actionpack4.0.gemfile.lock
lib/country_select.rb
lib/country_select/countries.rb
lib/country_select/version.rb
spec/country_select_spec.rb
spec/spec_helper.rb
", "spec/country_select_spec.rb
spec/spec_helper.rb
"] are not files
```

Calling `split('\n')` wasn't working since it should've been using a double-quoted string.