Bug converting proj string to cartopy for pyproj >= 2.0

[nickhand]

ALL software version info (Panel, bokeh, python, notebook, OS, browser, any other relevant packages)

geoviews 1.6.5 py_0 conda-forge
pyproj 2.3.1 py37h9bb365a_0 conda-forge

Description of expected behavior and the observed behavior

I believe this should be the same bug as holoviz/hvplot#312 with the PR fix in holoviz/hvplot#312. Related to the pyproj version.

I will open a PR with that fix in holoviz/hvplot#312 shortly.

Complete, minimal, self-contained example code that reproduces the issue

>>> import geoviews as gv
>>> gv.util.proj_to_cartopy("'+init=epsg:32611'")

Stack traceback and/or browser JavaScript console output

<ipython-input-4-00d09a57f4b9> in <module>
----> 1 gv.util.proj_to_cartopy("'+init=epsg:32611'")

~/Research/Programs/geoviews/geoviews/util.py in proj_to_cartopy(proj)
    456     #    if proj.crs.is_geographic:
    457     #        return ccrs.PlateCarree()
--> 458     if proj.is_latlong(): # pyproj < 2.0
    459         return ccrs.PlateCarree()
    460

Screenshots or screencasts of the bug in action