MAINT: stats: alias the erlang to the gamma distribution.  Add gamma._sf().

Closes gh-2172 and gh-198.  See discussion on gh-2172 for details.
MAINT: stats: Modify erlang to generate a RuntimeWarning when a non-integer shape parameter is used.
Merge PR-2507 from 'rgommers/erlang-alias-to-gamma' into scipy/master.

https://github.com/scipy/scipy/pull/2507