Use pipenv to build and test lizzy

[jmcs]

• Use Pipenv for (Zalando internal) CDP builds
• Use Pipenv for Travis.ci
• Use Pipenv for docker build
• Remove files that are no longer needed

[jmcs]

This will fix the issue we currently have on the master build.

[jmcs]

:+1:

[ethercrow]

:+1: