Upgrade rspec and fix warnings

[ivoanjo]

This PR:

• Upgrades to the latest rspec version
• Upgrades the test syntax to match the rspec 3 syntax with no deprecation warnings
• Enables a number of optional recommended rspec features
• Enables warnings during spec execution
• Solves all warnings in the gem code
• Refreshes the dependencies on the Gemfile.lock
• Adds ruby 2.2, 2.3 and JRuby 9k to the travis configuration file
• (some other tiny extras)

All specs still green, and I tested on both mri and jruby! Happy holidays!

[ivoanjo]

As travis was failing to install the latest versions of some of the test dependencies for 1.9 rubies, I took the liberty of dropping them from the test matrix. Give me a shout if you'd prefer to still support it.

[coveralls]

Coverage increased (+0.4%) to 98.8% when pulling 7d41c8f78788a215259ae4ac5d6d1d9775135139 on ivoanjo:upgrade-tests-fix-warnings into e03db5982f02933f0796e7086285931ad62d158d on hannesg:master.

[coveralls]

Coverage increased (+0.4%) to 98.8% when pulling 40b2565e13f12a6a0aedbca0108997d23a8fb488 on ivoanjo:upgrade-tests-fix-warnings into e03db5982f02933f0796e7086285931ad62d158d on hannesg:master.

[ivoanjo]

Any news? :)

[miguel-botelho]

Hey @hannesg ! 👋 I've just noticed this long standing PR after opening mine (https://github.com/hannesg/uri_template/pull/24), can I get a review on any of them? 🙏 It would be greatly appreciated, happy hacktoberfest 🎉