Make the build reproducible

[sebastic]

Overview

As reported in Debian Bug #1150326:

Whilst working on the Reproducible Builds effort, we noticed that pywps could not be built reproducibly.

This is because the documentation embeds the current build year into the documentation. A patch is attached that uses SOURCE_DATE_EPOCH if available.

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

coverage: 80.672%. remained the same — sebastic:reproducible-builds into geopython:main