These Installation instructions explain what to do when you encounter the new "Untrusted Enterprise Developer" message.

Steps:

1. Go to Settings > General

2. Scroll to the bottom, select Profile (this may also be called “Device Management”)

3. You can see the the untrusted developer’s name

4. Press on the developer’s name, Press Trust

5. Press Trust again

6. You will now be able to run the app.