TST: skip tests when using an old scipy for gaussian_kde
These tests are all marked slow, so not sure if they are being tested on travis. I just enabled them on windows, bunch fail (also some in tests/test_graphics.py), same reason

need to do an additional check not only skip if we don't have scipy, but must have a recent version (not exactly sure when gaussian_kde was added).

using scipy 0.10.1

```
======================================================================
ERROR: test_secondary_kde (pandas.tseries.tests.test_plotting.TestTSPlot)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\tseries\tests\test_plotting.py", line 573, in test_secondary_kde
    ax = ser.plot(secondary_y=True, kind='density').right_ax
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\tools\plotting.py", line 2256, in plot_series
    plot_obj.generate()
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\tools\plotting.py", line 902, in generate
    self._make_plot()
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\tools\plotting.py", line 1346, in _make_plot
    from scipy.stats import gaussian_kde
  File "C:\python26-32\lib\site-packages\scipy\stats\__init__.py", line 322, in <module>
    from stats import *
  File "C:\python26-32\lib\site-packages\scipy\stats\stats.py", line 198, in <module>
    import distributions
  File "C:\python26-32\lib\site-packages\scipy\stats\distributions.py", line 14, in <module>
    from scipy import optimize
  File "C:\python26-32\lib\site-packages\scipy\optimize\__init__.py", line 135, in <module>
    from nonlin import *
  File "C:\python26-32\lib\site-packages\scipy\optimize\nonlin.py", line 118, in <module>
    import scipy.sparse.linalg
  File "C:\python26-32\lib\site-packages\scipy\sparse\__init__.py", line 182, in <module>
    from csgraph import *
  File "C:\python26-32\lib\site-packages\scipy\sparse\csgraph\__init__.py", line 148, in <module>
    from ._shortest_path import shortest_path, floyd_warshall, dijkstra,\
ImportError: cannot import name shortest_path
```
