TST: use non-interactive matplotlib backend

Fixes a segfault at the end of `scipy.test()` on Python 3.4 for Windows when matplotlib 1.3.1 is installed and `TkAgg` is the default backend. Please consider backporting to version 0.14.x.

```
Running unit tests for scipy
NumPy version 1.8.1
NumPy is installed in X:\Python34\lib\site-packages\numpy
SciPy version 0.14.0rc2
SciPy is installed in X:\Python34\lib\site-packages\scipy
Python version 3.4.0 (v3.4.0:04f714765c13, Mar 16 2014, 19:25:23) [MSC v.1600 64 bit (AMD64)]
nose version 1.3.1
<snip>
----------------------------------------------------------------------
Ran 16416 tests in 193.865s

OK (KNOWNFAIL=277, SKIP=1157)
alloc: invalid block: 0000000005D4FE00: 60 5
```