Update versioning in _metadata.py
Update versioning in _metadata.py

Now using get_git_devstr in astropy_helpers to append the commit
number to the version number for development versions.  For example,
if we are at commit number 454 and working toward version 0.1, then
plasmapy.__version__ will be '0.1.dev454'.

StanczakDominik/PlasmaPy