Fix failing functional test for coverage plugin

Coverage plugin has extra requirements (cov-core>=1.12; see setup.py) which are not installed for tox and CI tests. This pull request makes requirements for test envs match those in setup.py.


PS. The failure in question is:
```
======================================================================
FAIL: test_run (nose2.tests.functional.test_coverage.TestCoverage)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "nose2\tests\functional\test_coverage.py", line 19, in test_run
    stderr=os.path.join('lib', 'mod1').replace('\\', r'\\') + STATS)
  File "nose2\tests\_common.py", line 64, in assertTestRunOutputMatches
    testf(util.safe_decode(cmd_stderr), stderr)
AssertionError: Regexp didn't match: 'lib/mod1           8      5    38%' not found in u'test1 (test_coverage.TestLib) ... ok\n\n----------------------------------------------------------------------\nRan 1 test in 0.004s\n\nOK\n----------- coverage: platform win32, python 2.7.6-final-0 -----------\nName           Stmts   Miss  Cover\n----------------------------------\nlib\\__init__       0      0   100%\nlib\\mod1           8      5    38%\n----------------------------------\nTOTAL              8      5    38%\n'
```