Fix typo in command to log pip-diff failure
Fix typo in command to log pip-diff failure

The pip-diff tool from vendor/pip-pop is used to determine stale
requirements. When pip-diff encounters an unexpected failure, a count is
logged using mcount from heroku/buildpack-stdlib.

Due to a typo, mount(8) was invoked instead of mcount, with an invalid
argument.

benmurden/heroku-buildpack-python-ember,AltSchool/heroku-buildpack-python,AltSchool/heroku-buildpack-python,heroku/heroku-buildpack-python,heroku/heroku-buildpack-python,skilljar/heroku-buildpack-python,Scalingo/python-buildpack,benmurden/heroku-buildpack-python-ember,Scalingo/python-buildpack