Fixed a minor bug in the get_pandas_dataframe function of Result

[JamesAllingham]

The loss_fn lambda was not being used and as a result, only the losses and not any info can be retrieved, for e.g by saying loss_fn = lambda r: {"val_NLL": r.loss, "val_err": r.info['valid err']}.

This PR does not change the default behaviour.

[codecov-io]

Codecov Report

Merging #86 into master will decrease coverage by 0.37%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   65.16%   64.78%   -0.38%     
==========================================
  Files          28       28              
  Lines        1846     1846              
==========================================
- Hits         1203     1196       -7     
- Misses        643      650       +7