Replace easy_install usage with the pip equivalents

[steveteahan]

easy_install (used with setup.py install) will install pre-releases
which is unexpected and undesirable behavior.

More information:

pypa/setuptools#855

I tested this by creating a venv with python setup.py install and another with pip install . The diff of the pip freeze output was:

$ diff easy_install.txt pip.txt 
29c29
< jira==2.0.1.0rc1
---
> jira==2.0.0
56c56
< python-jenkins==1.8.0.0a0
---
> python-jenkins==1.7.0
86c86
< wrapt==1.13.0rc3
---
> wrapt==1.12.1

This is exactly what I expect. The pip install ignored alpha and release candidates while easy_install was happy to install versions that we don't want to rely on in production.

[steveteahan]

I also built the container before and after this fix with dockerfiles/Dockerfile and confirmed that pre-release packages were being installed for jira, python-jenkins, and wrapt. With this fix, we're now using the stable releases.

[Piojo]

LGTM.

[jmelis]

LGTM