Run "pip check" on Travis for stricter dependency checking
Run "pip check" on Travis for stricter dependency checking

"pip check" will make sure that the package versions required by Pombola
and its dependencies are consistent - "pip install -r requirements.txt" is
actually happy to install those versions specified in requirements.txt
even if they conflict with requirements of dependent packages, but this
might well cause problems.

This pip subcommand was only introduced in pip 9.0.0, which is later
than the version you get by default on Travis, so use pip to upgrade
itself before running any other pip commands.

The --use-mirrors option has been removed in pip 7.0.0, so don't attempt
to use it any more.

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