I have ran into this issue twice now and have had to Google for it everytime. I thought i might as well post a little note here so i can remember it until next time. The solution to the problem is explained in this blogpost.
You have a few options but i have used the Powershell script you can find here and then just run it like this.
C:\PS> .\RemoveMasterKey.ps1 -bacpacPath "C:\BacPacs\Test.bacpac"
Well, that's it :)