Update travis config pending dep fixes

[ejolly]

Just opening an issue keep track of 2 bugs making testing on travis annoying right now:

1. conda update conda tries to uninstall setuptools for some reason and fails because setuptools is a conda dependency....weird. Here's an example of other folks encountering this. For the time being conda update conda has been removed form the before_install section of the .travis.yml since we create a new environment for testing anyway.

2. cffi a dependency of rpy2 was updated to 1.13 on conda-forge and is breaking rpy2>3. This is the the rpy2 bug for this, and cffi 1.13 breaks rpy2 3.1.0 on R 3.6 and Python 3.7 conda-forge/rpy2-feedstock#35 is the issue tracking the proposed fix