Use a later ruby in travis

[johnynek]

No description provided.

[codecov-io]

Codecov Report

Merging #650 into develop will decrease coverage by 0.11%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #650      +/-   ##
===========================================
- Coverage    82.85%   82.73%   -0.12%     
===========================================
  Files          109      108       -1     
  Lines         5179     5178       -1     
  Branches       462      314     -148     
===========================================
- Hits          4291     4284       -7     
- Misses         888      894       +6

Impacted Files	Coverage Δ
.../main/scala/com/twitter/algebird/Successible.scala	87.5% <0%> (-4.17%)	⬇️
.../main/scala/com/twitter/algebird/JavaMonoids.scala	62.9% <0%> (-1.62%)	⬇️
.../scala/com/twitter/algebird/MetricProperties.scala
.../main/scala/com/twitter/algebird/BloomFilter.scala	94.84% <0%> (+0.42%)	⬆️
.../main/scala/com/twitter/algebird/Applicative.scala	64.51% <0%> (+3.22%)	⬆️
...main/scala/com/twitter/algebird/monad/Reader.scala	58.82% <0%> (+5.88%)	⬆️
...src/main/scala/com/twitter/algebird/Interval.scala	86.08% <0%> (+6.95%)	⬆️

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[piyushnarang]

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