message: Attempt to fix docs build.
diff: @@ -12,6 +12,8 @@ fi
  if [ "$DOCS" = "yes" ]; then
  cargo install mdbook --no-default-features
  cargo install mdbook-linkcheck
  + sudo apt-get install python-pip
  + sudo pip install python-dateutil
  fi
  if [ "$TRAVIS_OS_NAME" = "osx" ] && [ "$GNUPLOT" = "yes" ]; then
  