gwpy-plot: Backend setting prevents headless ops

Created by: areeda

Error is:

This call to matplotlib.use() has no effect because the backend has already
been chosen; matplotlib.use() must be called *before* pylab, matplotlib.pyplot,
or matplotlib.backends is imported for the first time.

The backend was *originally* set to u'TkAgg' by the following code:
  File "/usr/local/ldvw/bin/gwpy-plot", line 32, in <module>
    from gwpy import __version__

Fix on the way.