Remove ruby 2.0 and add more recent versions to CI

[jbolinger]

CI is currently failing on Ruby 2.0 but I don't know if this version was ever supported anyway (the docs say 2.2). Should we keep testing 2.0? I also added newer versions.

[codecov[bot]]

Codecov Report

Merging #144 into master will increase coverage by <.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   99.22%   99.22%   +<.01%     
==========================================
  Files          19       19              
  Lines        2064     2069       +5     
==========================================
+ Hits         2048     2053       +5     
  Misses         16       16

Impacted Files	Coverage Δ
lib/google/gax/bundling.rb	99.37% <0%> (ø)	:arrow_up:
lib/google/gax/api_callable.rb	97.74% <0%> (+0.01%)	:arrow_up:
lib/google/gax/util.rb	97.82% <0%> (+0.04%)	:arrow_up:
lib/google/gax/grpc.rb	93.75% <0%> (+0.27%)	:arrow_up:

---

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[codecov[bot]]

Codecov Report

Merging #144 into master will increase coverage by 0.05%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   99.22%   99.27%   +0.05%     
==========================================
  Files          19       19              
  Lines        2064     2069       +5     
==========================================
+ Hits         2048     2054       +6     
+ Misses         16       15       -1

Impacted Files	Coverage Δ
lib/google/gax/bundling.rb	99.37% <0%> (ø)	:arrow_up:
lib/google/gax/api_callable.rb	97.74% <0%> (+0.01%)	:arrow_up:
lib/google/gax/util.rb	97.82% <0%> (+0.04%)	:arrow_up:
lib/google/gax/grpc.rb	93.75% <0%> (+0.27%)	:arrow_up:
spec/google/gax/bundling_spec.rb	100% <0%> (+0.3%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 20b7fca...cfd7fe9. Read the comment docs.