Travis: use Conda environments

Fix for ContinuumIO/anaconda-issues#6615

.travis.yml

@@ -28,7 +28,8 @@ before_install:
 
 # Install packages
 install:
-  - conda install --yes python=$TRAVIS_PYTHON_VERSION pip numpy scipy
+  - conda create --yes --name=$TRAVIS_PYTHON_VERSION python=$TRAVIS_PYTHON_VERSION numpy scipy
+  - source activate $TRAVIS_PYTHON_VERSION
   - pip install -r requirements.txt
   - pip install coveralls
   - pip freeze