Update Ruby version in travis

[anish749]

The builds via travis are failing because of an outdated ruby version which is no longer supported.
updating to the latest version passes the builds.

[codecov-io]

Codecov Report

Merging #674 into develop will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #674      +/-   ##
===========================================
+ Coverage    89.34%   89.36%   +0.02%     
===========================================
  Files          113      113              
  Lines         8944     8944              
  Branches       490      490              
===========================================
+ Hits          7991     7993       +2     
+ Misses         953      951       -2

Impacted Files	Coverage Δ
...ala/com/twitter/algebird/ApproximateProperty.scala	72% <0%> (-10%)	⬇️
.../main/scala/com/twitter/algebird/BloomFilter.scala	93.8% <0%> (+0.88%)	⬆️
.../main/scala/com/twitter/algebird/JavaMonoids.scala	64.51% <0%> (+1.61%)	⬆️
...src/main/scala/com/twitter/algebird/Interval.scala	77.39% <0%> (+2.6%)	⬆️
...main/scala/com/twitter/algebird/monad/Reader.scala	55.55% <0%> (+5.55%)	⬆️

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[johnynek]

Thank you for fixing the build!