Matplotlib deprecation warning

This warning is happening on import. We should remove the warn.

bilby/core/utils.py:966: MatplotlibDeprecationWarning: The 'warn' parameter of use() is deprecated since Matplotlib 3.1 and will be removed in 3.3.  If any parameter follows 'warn', they should be pass as keyword, not positionally.
  matplotlib.use(backend, warn=False)