get traitlets from requirements.txt on travis

needed for now, while various projects are unreleased

requirements.txt

@@ -0,0 +1,2 b''
+-e git+https://github.com/ipython/ipython_genutils.git#egg=ipython_genutils
+-e git+https://github.com/ipython/traitlets.git#egg=traitlets

.travis.yml

@@ -21,7 +21,7 b' before_install:'
     - git clone --quiet --depth 1 https://github.com/minrk/travis-wheels travis-wheels
     - 'if [[ $GROUP != js* ]]; then COVERAGE="--coverage xml"; fi'
 install:
-    - pip install -f travis-wheels/wheelhouse -e file://$PWD#egg=ipython[all] coveralls
+    - pip install -f travis-wheels/wheelhouse -r requirements.txt -e file://$PWD#egg=ipython[all] coveralls
 before_script:
     - 'if [[ $GROUP == js* ]]; then python -m IPython.external.mathjax mathjax.zip; fi'
 script: