install pyinstaller in linux

.travis.yml

@@ -68,6 +68,8 @@ install:
       # library but with a weird file name.  Details:
       # https://github.com/pyinstaller/pyinstaller/blob/61b1c75c2b0469b32d114298a63bf60b8d597e37/PyInstaller/hooks/hook-shapely.py#L34
       pip install --no-binary shapely -r requirements.txt
+
+      pip install pyinstaller
     
     elif [ "$TRAVIS_OS_NAME" = "osx" ]; then
       brew update