Several steps were made to achieve mutant working with Ruby 2.4 without
warnings and spec failures:
* Fix Fixnum deprecation warning
* Update RuboCop to 0.47 and fix offenses
* Relax version requirements on parser gem
* Update diff-lcs to 1.3
* Update rake 11.x -> 12.x
* Update reek 4.5 -> 4.6 (to work with parser 2.4)
* Update json 2.0 -> 2.1
* Update parallel 1.10 -> 1.11
* Update simplecov 0.12 -> 0.14
* Run regexp_parser integration against v0.4.3 (as we requiring this
exact version)
* Update warnings.yml with new whitelist
* Run CircleCI tests on Ruby 2.3 and 2.4 (Bundler.with_clean_env
was causing troubles on new CircleCI 2.0 setup, so it was removed)
