text backgroundcolor not clipped in figures

Hi!

``` python
import matplotlib.pyplot as plt
plt.text(0.9, 0.2, 'This is a bug?', backgroundcolor='r', clip_on=True)
plt.show()
```

The text itself is clipped but the background color of the text is not.

```
>>> matplotlib.__version__
'1.3.1'
```

I had difficulties to install 1.4.0 so I did not test it on 1.4.0

Thank you

[TAC edited for markup]
