Fix travis build definition for conditional dependencies

[cedricziel]

This change adjusts the before_install section to correctly install the optional dependencies.

Before this commit, the if statements silently failed (you can see that in the older build-logs), and the corresponding tests were skipped.

The patch ensures that the dependencies for ext-mongo, the odm and flysystem are installed so that the tests aren't skipped anymore.

[robfrawley]

Thanks for the PRs! Looks like the issue you are targeting was already addressed in #792.

[cedricziel]

Ahh, snap! :)

Thank you for the hint.