Remove dependency on io-console gem

`io/console` part of Ruby and coolline itself works without the gem. As
it turns out, this gem dependency is actually unnecessary and is
preventing pry-coolline from building under Ruby 2.0 (as the io-console
gem has native extensions that fail to configure). Remove the gem
dependency and instead use the `io/console` that already ships with Ruby
1.9.x to also support Ruby 2.0.0

Signed-off-by: David Celis <me@davidcel.is>
Update coolline dependency to ~> 0.4.0

Signed-off-by: David Celis <me@davidcel.is>
Make pry-coolline.gemspec a valid gemspec

The `test/test.rb` file no longer exists and was replaced long ago.
Building the gem locally throws warnings about the gemspec being valid
for that reason. The test files probably don't need to be included in
s.files, but s.test_files should include the real test files.

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #13 from davidcelis/remove-console-io

Remove console io
Merge pull request #14 from davidcelis/valid-gemspec

Make pry-coolline.gemspec a valid gemspec