Updates the mac build script so it wont fail when the venv folder does not exist.

script/build-osx

@@ -1,6 +1,6 @@
 #!/bin/bash
 set -ex
-rm -r venv
+rm -rf venv
 virtualenv venv
 venv/bin/pip install pyinstaller==2.1
 venv/bin/pip install .