Update bundler to 2.0 in travis builds

Author: timlah

.travis.yml

@@ -1,13 +1,17 @@
-language: ruby
-rvm:
-- 2.3.1
-install:
-- bundle install
-deploy:
-  provider: pages
-  skip_cleanup: true
-  github_token: $GITHUB_TOKEN
-  local_dir: _site
-  target_branch: master
-  on:
+language: ruby
+cache: bundler
+rvm:
+- 2.3.1
+before_install:
+  - gem update --system
+  - gem install bundler
+install:
+- bundle install
+deploy:
+  provider: pages
+  skip_cleanup: true
+  github_token: $GITHUB_TOKEN
+  local_dir: _site
+  target_branch: master
+  on:
     branch: develop