Fix Travis CI

.travisci/install.sh

@@ -2,6 +2,9 @@
 
 set -e
 
+# Workaround homebrew bug, see https://twitter.com/axccl/status/1083393735277363205 and https://github.com/openPMD/openPMD-api/pull/431/files
+brew upgrade --cleanup || brew upgrade --cleanup
+
 # Install newer GCC if we're running on GCC
 if [ "${CXX}" == "g++" ]; then
     # We need to uninstall oclint because it creates a /usr/local/include/c++ symlink that clashes with the gcc5 package