Uninstall bundler before installing the right one (#58)

- Remove bundler from global gemset prior to installing the correct version
- Add ruby 2.6 to the travis run

Author: arnoFleming

.travis.yml

@@ -4,8 +4,9 @@ rvm:
 - 2.3
 - 2.4
 - 2.5
+- 2.6
 - jruby-9.0
-before_install: gem install bundler -v 1.16.1
+before_install: echo Y | rvm @global do gem uninstall bundler ; gem install bundler -v 1.16.1
 cache: bundler
 matrix:
   allow_failures: