Merge pull request #312 from inkstitch/lexelby/pin-pyinstaller

pin to pyinstaller 3.3.1

.travis.yml

@@ -74,7 +74,7 @@ install:
       # https://github.com/pyinstaller/pyinstaller/blob/61b1c75c2b0469b32d114298a63bf60b8d597e37/PyInstaller/hooks/hook-shapely.py#L34
       pip install --no-binary shapely -r requirements.txt
 
-      pip install pyinstaller
+      pip install pyinstaller==3.3.1
     elif [ "$BUILD" = "windows" ]; then
       set -x