Rewrite install script for travis

[noha]

It seems that matrix builds in travis can not have a configurable before_install section. The current install does a mongo installation by default. If arango DB comes into play we need something else. Best approach so far would be to unite the DB install scripts that reacts on env variables to install one or the other database.

[gcotelli]

We have to do something similar to what are you proposing to test several database backends: https://github.com/ba-st/Sagan/blob/release-candidate/.travis.yml . It's not the most beautiful thing but just works.