only ask for coverage when it works (not js)

.travis.yml

@@ -22,11 +22,12 b' before_install:'
     - sudo apt-get install pandoc casperjs libzmq3-dev
     - git clone --quiet --depth 1 https://github.com/minrk/travis-wheels travis-wheels
     - 'if [[ $GROUP == js* ]]; then python -m IPython.external.mathjax; fi'
+    - 'if [[ $GROUP != js* ]]; then COVERAGE="--coverage xml"; fi'
 install:
     - pip install coveralls
     - pip install -f travis-wheels/wheelhouse file://$PWD#egg=ipython[all]
 script:
-    - cd /tmp && iptest $GROUP --coverage xml && cd -
+    - cd /tmp && iptest $GROUP $COVERAGE && cd -
 
 matrix:
     exclude: