TST: Fix tseries.converter test for MPL1.4
Closes #7233.

Other tests have no problem with MPL1.4.dev:
- tests\test_graphics.py
- tseries\tests\test_plotting.py

@cpcloud can we add mpl 1.4rc to the conda build? (not sure if you can do that)
 @sinhrks this is good thanks
 @sinhrks pls rebase and lmk when green (as I fixed the options error issue)
 I'm looking into getting a way for us to test pandas against dev versions of anything we want (they'll have to be conda packages but that not a big deal)
 @jreback Rebased, and now green.
