Removed the dev requirement of doctrine/mongodb-odm

This package is now added dynamically on Travis for supported PHP
versions (instead of being removed for HHVM). This avoids having a
dependency to ext-mongo when installing the dev dependencies of the
library, making it easier to run the testsuite for contributors.
Merge pull request #156 from stof/soft_mongo_deps

Removed the dev requirement of doctrine/mongodb-odm