Strip out all -e flags when processing requirements file
Strip out all -e flags when processing requirements file

Previously, there were various with installing editable packages in
different python / pip versions.
https://github.com/UnitedIncome/serverless-python-requirements/issues/240

This adds logic to strip out all '-e' editable flags from the
requirements.txt file and issues a warning to the CLI.

fixes #240

UnitedIncome/serverless-python-requirements,UnitedIncome/serverless-python-requirements