Use this numpy version

Author: jzuhone

.travis.yml

@@ -62,6 +62,7 @@ matrix:
         - env: CONDA_DEPENDENCIES="matplotlib=3 aplpy yt bottleneck joblib $CONDA_DEPENDENCIES"
                CONDA_CHANNELS="conda-forge $CONDA_CHANNELS"
                ASTROPY_VERSION='development'
+               NUMPY_VERSION=1.15
           name: vistools
 
         - env: # separate joblib-only test