Travis-ci: revert the bundler cache part of previous commit

.travis.yml

@@ -4,20 +4,13 @@ language: ruby
 
 sudo: false
 
-before_install:
-  - travis_retry gem install bundler
-
-cache:
-  - bundler
-
-bundler:
-  rvm:
+rvm:
   - 2.2.3
   - 2.1
   - 2.0
   - 1.9
 
-  env:
+env:
   - RAILS=4-2-stable DB=mongodb
   - RAILS=4-2-stable DB=sqlite3
   - RAILS=4-2-stable DB=mysql
@@ -40,7 +33,7 @@ bundler:
   - RAILS=3-1-stable DB=mysql
   - RAILS=3-1-stable DB=postgres
 
-  matrix:
+matrix:
   include:
     - rvm: 2.2
       env: RAILS=master DB=sqlite3