Pin pytest-cov version to fix Travis build

[vshlapakov]

The best description of the problem is given by pywbem/pywbem#1371 (comment):

Analysis shows that this is related to the issue around requiring different versions of the coverage package: pytest-cov 2.6.0 has increased the version requirement for the coverage package from >=3.7.1 to >=4.4, which is in conflict with the version requirement defined by the python-coveralls package for coverage==4.0.3. Issue z4r/python-coveralls#66 has been opened. Workaround is to pin the pytest_cov version to below 2.6.0.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1da1f70). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #107   +/-   ##
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  Coverage          ?   93.42%           
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  Files             ?       28           
  Lines             ?     1885           
  Branches          ?        0           
=========================================
  Hits              ?     1761           
  Misses            ?      124           
  Partials          ?        0

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