Travis: use jruby-9.1.15.0#23

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olleolleolle wants to merge 3 commits intoapotonick:masterfrom

olleolleolle:patch-1

Commits on Sep 12, 2017

Travis: use jruby-9.1.13.0
olleolleolle
authored

Commits on Nov 12, 2017

Travis: jruby-9.1.14.0
olleolleolle
authored

Commits on Dec 15, 2017

Travis: jruby-9.1.15.0
olleolleolle
authored