Drop support for ruby < 2.3.0 and update dev dependencies

[jfeltesse-mdsol]

Ruby 2.2 has reached EOL. Technically this gem is obviously still compatible with earlier versions so it's only about whether we want to support EOL rubies and the version the users of this gem are using.
AFAIK at medidata all ruby projects that are maintained and developed are at least at 2.3.x.

At the same time I cleaned up the dev dependencies:

• updated where applicable
• removed kender because it was not used
• removed uuidtools because securerandom does the job just fine now

I also cleaned up the travis file, the fixes around bundler are not needed anymore.

About the semver, doing a major vs minor bump in this case is subject to interpretation.
I think this discussion on nokogiri's repo brings some good points in favor of not bumping major.

@mdsol/team-10 @ejennings-mdsol @johngluckmdsol

[ssteeg-mdsol]

Looks good to me 👍

[piao-mdsol]

👍 LGTM! And thanks for linking to the Nokogiri issue, bumping the version sounds reasonable to me too ✌️