CI: Ensure --pre flag applies to all dependencies

Author: effigies

.travis.yml

@@ -63,7 +63,7 @@ before_install:
     export FSLOUTPUTTYPE=NIFTI_GZ;
   fi;
 
-- travis_retry pip install -r requirements.txt
+- travis_retry pip install $EXTRA_PIP_FLAGS -r requirements.txt
 - travis_retry pip install grabbit==0.2.6
 - travis_retry pip install -e git+https://github.com/bids-standard/pybids.git@0.7.0#egg=pybids