Ruby 2.0 compatibility: remove implicit conversion of nil into String
Update Gemfile to use HTTPS requests

The source :rubygems is deprecated because HTTP requests are insecure.
Add Ruby 2.0 to Travis
code style cleanup in models
Merge pull request #418 from ifarkas/ruby2.0-compatibality

Fix Ruby 2.0 compatibility issues
Merge pull request #422 from martinpovolny/coding_style

code style cleanup in models