test_boxplot catche DeprecationWarning on jessie with python 3.4
```
======================================================================
FAIL: test_boxplot (pandas.tests.test_graphics.TestDataFramePlots)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python3/dist-packages/pandas/tests/test_graphics.py", line 1821, in test_boxplot
    subplots=True, logy=True)
  File "/usr/lib/python3.4/contextlib.py", line 66, in __exit__
    next(self.gen)
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python3/dist-packages/pandas/util/testing.py", line 1627, in assert_produces_warning
    % extra_warnings)
AssertionError: Caused unexpected warning(s): ['DeprecationWarning'].
```

```
python3.4 ci/print_versions.py; \
    cd build/; xvfb-run -a -s "-screen 0 1280x1024x24 -noreset" \
      python3.4 /usr/bin/nosetests -s -v -A 'not network and not disabled' --exclude test_bar_log  pandas;

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.2.final.0
python-bits: 64
OS: Linux
OS-release: 3.2.0-4-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: C
LANG: C

pandas: 0.15.0
nose: 1.3.4
Cython: 0.20.1post0
numpy: 1.8.2
scipy: 0.14.0
statsmodels: None
IPython: None
sphinx: 1.2.3
patsy: None
dateutil: 2.0
pytz: 2012c
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.3.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
```
