Drop support for Rubinius

[skalee]

Rubinius appears to be very difficult in testing. Builds take over five minutes. Cucumber tests tend to fail randomly, or produce weird output.

[codecov[bot]]

Codecov Report

Merging #80 into master will decrease coverage by 0.98%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage    99.6%   98.61%   -0.99%     
==========================================
  Files          11       10       -1     
  Lines         501      361     -140     
==========================================
- Hits          499      356     -143     
- Misses          2        5       +3

Impacted Files	Coverage Δ
lib/uri_format_validator/reacher.rb	87.5% <0%> (-4.17%)	:arrow_down:
...b/uri_format_validator/validators/uri_validator.rb	96.38% <0%> (-3.62%)	:arrow_down:
spec/uri_validator_spec.rb	100% <0%> (ø)	:arrow_up:
spec/reacher_spec.rb	100% <0%> (ø)	:arrow_up:
.../uri_format_validator/validators/helper_methods.rb	100% <0%> (ø)	:arrow_up:
spec/support/model.rb	100% <0%> (ø)	:arrow_up:
lib/uri_format_validator/validators.rb	100% <0%> (ø)	:arrow_up:
spec/helper_methods_spec.rb

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[ronaldtse]

Agree @skalee , thanks!