Upgrade Ruby versions and Relax dependancies

[berkos]

👋

• Upgrade to latest Rubies on Travis.
• Remove Gemfile.lock to relax dependancies between different Ruby versions.
• Fix offences in latest Rubocop. (Should the version of rubocop locked?)

note: Gemfile.lock is bundled with a specific version of bundler, not sure if there is a reason to keep the lock file as versions can locked from gemspec.

Thanks!

[coveralls]

Coverage decreased (-0.0006%) to 99.825% when pulling 572a552 on berkos:upgrade_Ruby_versions_on_travis into 22d06c8 on joost:master.

[joost]

Thanks. Useful changes.

[berkos]

Thanks for the response and merging the PR.
If we find that Rubocop keeps finding new offences when updates to the latest version we could lock Rubocop version as development dependancy.