Drop support for versions of rubies and Rails that no longer receive security updates, fix tests, support Rails 5.1

[glebm]

1. MRI rubies < 2.2 have not been maintained for over a year and no longer receive security fixes! Nobody should be supporting these versions anymore. https://www.ruby-lang.org/en/news/2017/04/01/support-of-ruby-2-1-has-ended/

   Updates .travis.yml to only test on latest PATCH versions for 2.2, 2.3, and 2.4. Relaxes jruby-9.1.5.0 to jruby-9, and changes rbx to rbx-3 (as rbx alone does not work at all on travis).

2. Rails < 4.2 no longer receive security fixes. http://rubyonrails.org/security/ Removes support for and testing with all versions of Rails < 4.2.
3. Removes Rails 5.0 + Sprockets 4.0.0.beta tests. Sprockets 4.0 beta is unstable (see changelog), so supporting it before an RC is probably not worth it.
4. Adds Rails 5.1 tests.
5. Tests now pass on Rubinius so removes rbx from allowed failures.

With this, #69 and #65 can be closed.

[codecov-io]

Codecov Report

Merging #70 into master will decrease coverage by 59.81%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           master      #70       +/-   ##
===========================================
- Coverage   95.99%   36.18%   -59.82%     
===========================================
  Files          21     2189     +2168     
  Lines         599   102571   +101972     
===========================================
+ Hits          575    37113    +36538     
- Misses         24    65458    +65434

Impacted Files	Coverage Δ
spec/support/rails_app.rb	93.87% <100%> (+2.57%)	:arrow_up:
...ems/nokogiri-1.7.1-java/lib/nokogiri/xml/schema.rb	0% <0%> (ø)
...0.5/lib/concurrent/atomic/ruby_thread_local_var.rb	31.34% <0%> (ø)
...ms/mail-2.6.5/lib/mail/fields/resent_from_field.rb	64.28% <0%> (ø)
.../gems/mail-2.6.5/lib/mail/fields/reply_to_field.rb	0% <0%> (ø)
...1.0/lib/active_support/core_ext/object/deep_dup.rb	43.75% <0%> (ø)
...s/nokogiri-1.7.1/lib/nokogiri/xml/xpath_context.rb	71.42% <0%> (ø)
...dle/ruby/2.4.0/gems/nokogiri-1.7.1/lib/nokogiri.rb	52.17% <0%> (ø)
...s/mail-2.6.5/lib/mail/fields/common/common_date.rb	62.5% <0%> (ø)
... and 2163 more

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[glebm]

@Mange I believe this addresses all of the critical/urgent issues. Let's merge this in and release a new version?

[Mange]

Thank you very much! All of this makes sense, and the commits have good motivations behind them. :heart:

[Mange]

I just released version 1.2.0 including this PR. Thank you again!