[CI,FIX] fix travis contrib clone problems

In some cases the travis build machines failed when trying to clone/build the contrib libraries. This pull request improves the behaviour allowing a second try for each individual build step. 

Additionally 

 * added checkout of search engines to enable search engine tests
 * reduced amount of installed dependencies to speed up build times
 * added `REPO_SLUG` to build name for easier identification of travis build on crash