Install nbformat in different ways, depending on python version.

Sigh.

Author: arokem

.travis.yml

@@ -25,7 +25,10 @@ before_install:
   - conda update conda
   - conda info -a
   - travis_retry conda create -n test $CONDA IPython pip nose pyzmq jsonschema
-  - travis_retry pip install nbformat
+  - if [[ $CONDA == python=3.3* ]]; then
+      pip install nbformat;
+    else
+      travis_retry conda install nbformat;
   - source activate test
   - travis_retry pip install coveralls