vagrant: upgrade pyinstaller to v3.5 + patch

the patch is needed so it works with recent virtualenv.

Vagrantfile

@@ -198,7 +198,7 @@ def install_pyinstaller()
     . borg-env/bin/activate
     git clone https://github.com/thomaswaldmann/pyinstaller.git
     cd pyinstaller
-    git checkout v3.3.1
+    git checkout v3.5-maint
     python setup.py install
   EOF
 end