Temporarily downgrade testpath on travis

I think this may solve the error at https://travis-ci.org/jupyterlab/jupyter-renderers/builds/437449573#L431. See conda-forge/testpath-feedstock#6 and conda-forge/testpath-feedstock#8 for details. Once testpath 4.2 is released, hopefully this is fixed.

.travis.yml

@@ -10,7 +10,9 @@ install:
 - conda config --set always_yes yes --set changeps1 no
 - conda update -q conda
 - conda info -a
-- conda install nodejs notebook
+- conda install nodejs notebook 'testpath<0.4'
+- which pip
+- pip --version
 - pip install --pre jupyterlab
 script:
 - yarn