[CI] Move the Bundler install command on Travis to the before_install section

Related: elastic#538

Author: karmi

.travis.yml

@@ -20,6 +20,11 @@ jdk:
 services:
   - mongodb
 
+before_install:
+  - gem update --system
+  - gem --version
+  - gem install bundler -v 1.11.2
+
 before_script:
   - ls -la /usr/share/elasticsearch/bin/elasticsearch
   - echo $PWD
@@ -29,8 +34,6 @@ before_script:
 script:
   - SERVER=launch TEST_CLUSTER_COMMAND=/usr/share/elasticsearch/bin/elasticsearch TEST_CLUSTER_PARAMS='-Des.default.path.conf=/etc/elasticsearch/ -Des.default.path.logs==/var/log/elasticsearch/' bundle exec rake test:all
 
-install: "gem uninstall bundler -axI && gem install bundler -v 1.11.2"
-
 notifications:
   disable: true