Update pipenv-to-pip to read lockfiles properly
Update pipenv-to-pip to read lockfiles properly

Use the appropriate encoding to read a Pipfile.lock so we can
properly load the data.

Closes #678
benmurden/heroku-buildpack-python-ember,edmorley/heroku-buildpack-python,benmurden/heroku-buildpack-python-ember,ad-m/heroku-buildpack-python,heroku/heroku-buildpack-python,heroku/heroku-buildpack-python,benjaminfox/heroku-buildpack-python-geoip,benjaminfox/heroku-buildpack-python-geoip,AltSchool/heroku-buildpack-python,ad-m/heroku-buildpack-python,skilljar/heroku-buildpack-python,AltSchool/heroku-buildpack-python,Scalingo/python-buildpack,joshfriend/heroku-buildpack-python,Scalingo/python-buildpack