.travis.yml in stellar-base-0.15.0 vs .travis.yml in stellar-base-0.16.0

@@ -1,9 +1,10 @@
 language: ruby
 rvm:
 - 2.2.6
 - 2.3.3
 - 2.4.0
+- 2.5.1
 script: bundle exec rake travis
 notifications:
   slack:
     secure: V/6a8KFe067uukrbCJA2R1HPO4xy0YSQ1pwmHVRi5StX+yl+lYsWWJdjdBdT0j3iJBYyPRqU4bQYck+OloxtELnrHCX+OkodxcxW8W/ACc914iIf0FyY9pnusK7ck2awmt4Iuf94YPgi0XTm1aCcm+f0yU7wiIVFpftoXSk1EDY=