pytest version was incompatible with astropy version

(aside: @#$%*)

Author: keflavich

.travis.yml

@@ -101,7 +101,7 @@ matrix:
           env: PYTHON_VERSION=3.5 NUMPY_VERSION=1.12  KEYRING_VERSION='<12.0'
                CONDA_DEPENDENCIES="keyring requests beautifulsoup4 html5lib secretstorage"
                SECRETSTORAGE_VERSION='<3.0.1'
-               ASTROPY_VERSION=2 PYTEST_VERSION='3.1' ASTROPY_USE_SYSTEM_PYTEST=1
+               ASTROPY_VERSION=2 PYTEST_VERSION='<3.7'
         - os: linux
           stage: Tests with other Python/Numpy versions
           env: PYTHON_VERSION=3.5.5 NUMPY_VERSION=1.12 KEYRING_VERSION='<12.0'