Merge pull request #5 from nubs/master

Rebase the Dockerfile off of nubs/phpunit.
Run multiple mongo db versions and extensions in the travis build.

Removed a php version to keep the number of runs down.
Merge pull request #6 from gaillard/master

Run multiple mongo db versions and extensions in the travis build.
Update predis to 1.0.

The [release notes](https://github.com/nrk/predis/releases/tag/v1.0.0)
are pretty clear about BC breaks and none of them appear to affect our
usage of the client here or elsewhere.