Install enum34 in Python 2.7 CI builds.
Install enum34 in Python 2.7 CI builds.

This is a workaround for `conda-build` 2.1.0 packages that are missing
the requirement on `enum34` in their Python 2.7 packages. It can be
reverted after that is fixed.

ocefpaf/conda-smithy,conda-forge/conda-smithy,shadowwalkersb/conda-smithy,conda-forge/conda-smithy,ocefpaf/conda-smithy,shadowwalkersb/conda-smithy