Remove the pin "proj4<5", introduced in #3150

Background:
#3150 fixed problems emerging with cartopy and Proj4 v 5.1
Apparently triggered by new builds on conda_forge (though also anaconda?).
A new build of cartopy 0.16, though no new version.

Needs fixing properly, but probably better done in cartopy ?
Possibly, cartopy may not work properly with Proj4 >= 5.
Which said, the newest build of cartopy 0.16 may be effectively invalid

• as here , lists as "py27h81b52dc_1 conda-forge",