Install xarray and dask in travis testing

Author: efiring

.travis.yml

@@ -36,7 +36,7 @@ before_install:
     conda create --name TEST python=$PY --file requirements-dev.txt --quiet
     source activate TEST
     # Install after to ensure it will be downgraded when testing an older version.
-    conda install numpy=$NUMPY
+    conda install numpy=$NUMPY xarray dask
     conda info --all
 
 # Test source distribution.
@@ -64,7 +64,7 @@ script:
       pushd docs
       make clean html linkcheck
       popd
-      if [[ -z "$TRAVIS_TAG" ]]; then 
+      if [[ -z "$TRAVIS_TAG" ]]; then
         python -m doctr deploy --build-tags --key-path github_deploy_key.enc --built-docs docs/_build/html dev
       else
         python -m doctr deploy --build-tags --key-path github_deploy_key.enc --built-docs docs/_build/html "version-$TRAVIS_TAG"