Here is a patch which you can run on Mac Office 15.11.2 (the preview version) to stop it from expiring. In other words it should convert the time-limited 15.11.2 preview version to the release version.

You will need to run this in Terminal. If you get errors because the MicrosoftSetupUI.framework is missing (check this in Terminal), you will be prompted to install that framework, or you can install it independently. Then re-run the patch and it should work fine. Worked for me in OSX 10.10.4 (Yosemite)