Fix "Legend.draggable() is deprecated" warning

[marko-bast]

Just a small fix for a deprecated warning.

Before:

$ python -m spinup.run plot data/installtest/installtest_s0 -s3
Plotting from...
==================================================

data/installtest/installtest_s0

==================================================
/anaconda3/envs/spinningup/lib/python3.6/site-packages/seaborn/timeseries.py:183: UserWarning: The tsplot function is deprecated and will be removed or replaced (in a substantially altered version) in a future release.
  warnings.warn(msg, UserWarning)
/anaconda3/envs/spinningup/lib/python3.6/site-packages/matplotlib/legend.py:1196: MatplotlibDeprecationWarning: 
Legend.draggable() is drepecated in favor of Legend.set_draggable(). Legend.draggable may be reintroduced as a property in future releases.
  message="Legend.draggable() is drepecated in "

After:

$ python -m spinup.run plot data/installtest/installtest_s0 -s3
Plotting from...
==================================================

data/installtest/installtest_s0

==================================================
/anaconda3/envs/spinningup/lib/python3.6/site-packages/seaborn/timeseries.py:183: UserWarning: The tsplot function is deprecated and will be removed or replaced (in a substantially altered version) in a future release.
  warnings.warn(msg, UserWarning)