Don't run `gem update --system` and `gem install bundler` on CI

There were some issues with the combination of some `bundler` and `rubygems` versions.
We got around that by always updating `bundler` in the CI but since those issues
were fixed we can quit doing that.

Related to `gem update --system`:
- travis-ci/travis-ci#8978
- rubygems/rubygems#2123

Related to `gem install bundler`:
- travis-ci/travis-ci#9333
- travis-ci/travis-ci#8969
- bundler/bundler#6340

.travis.yml

@@ -51,10 +51,7 @@ env:
     - DEVISE_ORM=active_record
     - DEVISE_ORM=mongoid
 
-before_install:
-  - gem update --system
-  - gem install bundler
-  - "rm ${BUNDLE_GEMFILE}.lock"
+before_install: "rm ${BUNDLE_GEMFILE}.lock"
 
 before_script: "bundle update"