.travis.yml in stellar-base-0.18.0 vs .travis.yml in stellar-base-0.19.0

@@ -1,14 +1,13 @@
 language: ruby
 rvm:
-- 2.2.6
-- 2.3.3
-- 2.4.0
 - 2.5.1
+- 2.6.2
 before_install:
   - sudo add-apt-repository -y ppa:chris-lea/libsodium
   - sudo apt-get -y update
   - sudo apt-get install -y libsodium-dev
+  - gem update bundler
 script: bundle exec rake travis
 notifications:
   slack:
     secure: V/6a8KFe067uukrbCJA2R1HPO4xy0YSQ1pwmHVRi5StX+yl+lYsWWJdjdBdT0j3iJBYyPRqU4bQYck+OloxtELnrHCX+OkodxcxW8W/ACc914iIf0FyY9pnusK7ck2awmt4Iuf94YPgi0XTm1aCcm+f0yU7wiIVFpftoXSk1EDY=