Add Cloud Foundry dependency to Travis (#221)

Deploying from CI failed because the `cf` command was not present on the CI server.

This commit adds Cloud Foundry as a dependency per the example in the [cloud.gov docs](https://docs.cloud.gov/apps/continuous-deployment/)

.travis.yml

@@ -3,6 +3,11 @@ before_script:
 - bin/rake db:setup --trace
 bundler_args: "--deployment --jobs=3 --retry=3 --without development"
 cache: bundler
+dependencies:
+  pre:
+  - wget http://go-cli.s3-website-us-east-1.amazonaws.com/releases/latest/cf-cli_amd64.deb -qO temp.deb
+  - sudo dpkg -i temp.deb
+  - cf -v
 env:
   global:
   - DB=postgresql