Possible test_plotting regression in 0.11
I'm getting failures on master. Travis full_deps should have caught this,
maybe something is screwy about my system. passes on 10.1.

check it out before releasing 0.11, fix travis if needs be.

``` python
nosetests ./pandas/tseries/tests/test_plotting.py
.....................EE..EEEEE........EE...........E.

======================================================================
ERROR: test_tsplot (pandas.tseries.tests.test_plotting.TestTSPlot)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/user1/src/pandas/pandas/tseries/tests/test_plotting.py", line 97, in test_tsplot
    _check_plot_works(f, s.index.freq, ax=ax, series=s)
  File "/home/user1/src/pandas/pandas/tseries/tests/test_plotting.py", line 967, in _check_plot_works
    with ensure_path() as path:
NameError: global name 'ensure_path' is not defined

```
