fix(travis): Run bower install twice to make sure it does

Like angular/angular.js is doing.

This is a work-around for Bower/Node.js issue (bower/bower#830). We run `bower install` twice, as the probability of failing twice in a row is very low.

.travis.yml

@@ -7,7 +7,8 @@ before_install:
 - sh -e /etc/init.d/xvfb start
 - npm install -g bower grunt-cli
 - npm install
-- bower install
+- bower install --force
+- bower install --force
 before_script: ./bower_components/angular-ui-docs/.travis/before_script.sh
 after_success: ./bower_components/angular-ui-docs/.travis/after_success.sh
 branches: