Install different python versions in separate environments.

Author: nathanlyons

.travis.yml

@@ -31,7 +31,8 @@ install:
 - export PATH="$CONDA_PREFIX/bin:$PATH"
 - hash -r
 - conda config --set always_yes yes --set changeps1 no
-- conda install python=$TRAVIS_PYTHON_VERSION
+- conda create -n _testing python=$TRAVIS_PYTHON_VERSION
+- source activate _testing
 - conda install scripting -c csdms-stack
 - conda install landlab jupyter -c landlab
 - conda info -a && conda list