(PUP-2948) enable rubocop for travisCI

Before this commit, rubocop was executed for each ruby platform in travisCI.
This commit enables rubocop only for the latest ruby platform (2.1.1). In a
nutshell, the changes included are

- Add rubocop step in travis CI
- Ensure rubocop is present in test group
- Ensure rubocop is not added for ruby 1.8