Ok cool. Got it
So I have followed what you have said, and It has come up with another error :S
Looks like it might be the same one, but I'm not sure. Still saying 'Dexer' amongst the errors.
You still have two Heyzap jar files and some old fyber jars.
1) Go to ../stencylworks/engine-extensions/heyzap/dependencies/heyzap/libs/
2) Delete: heyzap-unified-platform-10.2.0.jar, fyber-admob-9.4.0-r2.jar and fyber-hyprmx-4.0.1-r3.jar
3) You now have only heyzap-ads-sdk-9.10.1.jar inside the libs folder
4) Clean project (very important) and run it