cleanup travis file

Author: 3zcurdia

.travis.yml

@@ -1,12 +1,8 @@
 language: ruby
-before_install: gem install bundler && bundler -v
-install: bundle install --jobs=3 --retry=3
-before_script: bundle install
-script: bundle exec rake
 rvm:
-  - 2.1.8
-  - 2.2.4
   - ruby-head
-matrix:
-  allow_failures:
-    - rvm: ruby-head
+  - 2.4.3
+  - 2.3.6
+  - 2.2.9
+  - 2.1.9
+before_install: gem install bundler -v 1.16.0