CI: Install pytest>=3.4 in Travis

Author: effigies

.travis.yml

@@ -53,6 +53,7 @@ before_install:
     export FSLOUTPUTTYPE=NIFTI_GZ;
   fi;
 
+- travis_retry pip install --upgrade pytest>=3.4  # Work around pip failure
 - travis_retry pip install -r requirements.txt
 - travis_retry pip install grabbit==0.1.2
 - travis_retry git clone https://github.com/INCF/pybids.git ${HOME}/pybids &&