update plone api versions/bootstrap process

[thet]

• use plone 5.1
• do not use bootstrap.py but install zc.buildout via pip (see: bootstrap.sh)

[coveralls]

Coverage remained the same at 58.211% when pulling acfa140 on thet-updapi into 7852c9f on master.

[tisto]

@thet thanks for the upgrade. Two remarks from my side. I'd keep the bootstrap-buildout.py for people who prefer that approach. I'd also prefer a Makefile over a bash-script, because you do not risk to end up with a bunch of bash-scripts in your root folder.