TST/TEMP: fix pyqt to 4.x for plotting tests
To fix breaking tests (latest travis build on master broken, and also all recent PR builds), see https://github.com/matplotlib/matplotlib/issues/7124

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14240?src=pr) is 85.24% (diff: 100%)

> No coverage report found for **master** at e596cbf.
> 
> Powered by [Codecov](https://codecov.io?src=pr). Last update [e596cbf...f961d59](https://codecov.io/gh/pydata/pandas/compare/e596cbf82410fcb19d7c8cbe2f22a8b588afd390...f961d594fbd52070bd124ed8efa0f514da920caa?src=pr)
 See https://github.com/ContinuumIO/anaconda-issues/issues/1068
