.travis.yml in tikkie-api-0.1.1 vs .travis.yml in tikkie-api-0.2.0

@@ -1,8 +1,11 @@
 sudo: false
 language: ruby
+cache: bundler
+before_install:
+  - gem update --system
+  - gem update bundler
 rvm:
-  - 2.2.10
-  - 2.3.7
-  - 2.4.4
-  - 2.5.1
-before_install: gem install bundler -v 1.16.1
+  - 2.3.8
+  - 2.4.5
+  - 2.5.3
+  - 2.6.1