fix keras backend=theano in travis config

.travis.yml

@@ -39,6 +39,6 @@ script:
   # download data and models, then run tests
   - mhcflurry-downloads fetch
   - mhcflurry-downloads info  # just to test this command works
-  - PYTHONHASHSEED=0 nosetests test --with-coverage --cover-package=mhcflurry  && ./lint.sh
+  - KERAS_BACKEND=theano PYTHONHASHSEED=0 nosetests test --with-coverage --cover-package=mhcflurry  && ./lint.sh
 after_success:
   coveralls