Fixed dependency versions so pyup will work

[s-weigand]

Sorry for the delay, but here is the PR promissed in #12 .
With this the PRs from pyup will indicate if updates from tox or pytest, will break the plugin.

[coveralls]

Pull Request Test Coverage Report for Build 30

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 28:	0.0%
Covered Lines:	291
Relevant Lines:	291

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💛 - Coveralls

[s-weigand]

The failing of the tests was due to our own tests using the caplog fixture (to capture printing), which was introduced to pytest in version 3.4.
But this shouldn't compromice the fuctionality since it's just a dev-tool, so i guess there is no need to set a min version of pytest in the setup.py.

[oz123]

/lgtm