Fixed regular expression in test for Plone version.

[mauritsvanrees]

Plone 5.1.0.1 gives a test failure on Jenkins, because the fourth level was not expected:

Regexp didn't match: '^(\\d+\\.\\d+|\\d+\\.\\d+\\.\\d+)(a\\d+|b\\d+|rc\\d+)?(\\.dev\\d)?$' not found in '5.1.0.1'

  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "/home/jenkins/.buildout/eggs/plone.api-1.8.2-py2.7.egg/plone/api/tests/test_env.py", line 434, in test_plone_version
    '^(\d+\.\d+|\d+\.\d+\.\d+)(a\d+|b\d+|rc\d+)?(\.dev\d)?$',
  File "/usr/lib/python2.7/unittest/case.py", line 1002, in assertRegexpMatches
    raise self.failureException(msg)

We could maybe use pkg_resources.parse_version or distutils or packaging. See stackoverflow for some options.
But I opted to change the regexp that we were using.

[hvelarde]

LGTM

[mauritsvanrees]

Travis fails, but it was already failing. This is the QA part.

This fixes Jenkins 5.1. I will merge now.