chore: update ruby & bundler version for CI

* bundler 1.12.5 breaks with current version of dependencies
  (NoMethodError: undefined method `[]' for nil:NilClass)
* vagrant-2.2.17 requires ruby version ~> 2.5, < 2.8

.travis.yml

@@ -1,10 +1,10 @@
+---
 language: ruby
 rvm:
-  - 2.2
-  - 2.3
-  - 2.4
   - 2.5
+  - 2.6
+  - 2.7
 install:
-  - gem install -v 1.12.5 bundler
-  - bundle _1.12.5_ install --jobs=3 --retry=3
+  - gem install -v 1.16.2 bundler
+  - bundle _1.16.2_ install --jobs=3 --retry=3
 script: "bundle exec rake ci"