BLD: _check_plot_works(df.plot) in test results in RuntimeError: Error building image
While building debian package for 0.11.0+git43-g7b2eaa4, matplotlib 1.1.1~rc2-1 and while building it should use Agg backend, that is how tests are invoked

```
python-test%: python-install%
ifeq (,$(filter nocheck,$(DEB_BUILD_OPTIONS)))
        echo "backend : Agg" >| $(CURDIR)/build/matplotlibrc
        : # Run unittests here against installed pandas
        echo "$*" | grep -q '^3' && PY=3 || PY=$*; \
        export PYTHONPATH=`/bin/ls -d $$PWD/debian/tmp/usr/lib/python$$PY/*/`; \
        export MPLCONFIGDIR=$(CURDIR)/build HOME=$(CURDIR)/build; \
        cd build/; python$* /usr/bin/nosetests -s -v -a '!network' $(EXCLUDE_TESTS$*) pandas;
else
        : # Skip unittests due to nocheck
endif
```

and here is a prototypical error (there is more for other invocations for the _check):

```
======================================================================
ERROR: test_line_plot_period_frame (pandas.tseries.tests.test_plotting.TestTSPlot)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/yoh/deb/gits/pkg-exppsy/build-area/pandas-0.11.0+git43-g7b2eaa4/debian/tmp/usr/lib/python2.7/dist-packages/pandas/tseries/tests/test_plotting.py", line 165, in test_line_plot_period_frame
    _check_plot_works(df.plot, df.index.freq)
  File "/home/yoh/deb/gits/pkg-exppsy/build-area/pandas-0.11.0+git43-g7b2eaa4/debian/tmp/usr/lib/python2.7/dist-packages/pandas/tseries/tests/test_plotting.py", line 1006, in _check_plot_works
    plt.savefig(path)
  File "/usr/lib/pymodules/python2.7/matplotlib/pyplot.py", line 471, in savefig
    return fig.savefig(*args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/figure.py", line 1185, in savefig
    self.canvas.print_figure(*args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/backend_bases.py", line 2021, in print_figure
    **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_agg.py", line 477, in print_png
    filename_or_obj, self.figure.dpi)
RuntimeError: Error building image
```
