Ignore nose plugins to prevent test timing issues

[hwkns]

When I upgraded from freezegun 0.3.9 to 0.3.10, my CircleCI builds started taking longer. Upon further inspection, the tests were being distributed very strangely across the available containers. This was because the xunit output about how long each test took to run was very wrong; one test that took a few ms to run was marked as taking 25494780 seconds. After realizing that value was being consistently reported every time, I thought time might be stopped, and sure enough, downgrading fixed my builds (as did this proposed change).

I'm not sure what is different between 0.3.9 and 0.3.10 that breaks the timing component of the built-in xunit plugin, so I can't offer a more targeted fix. Please consider this change to add nose.plugins to the ignore list.

[coveralls]

Coverage increased (+0.01%) to 94.17% when pulling afc9ddc on hwkns:ignore-nose-plugins into 5d02625 on spulec:master.

[spulec]

Sounds good, thanks.