Travis: Repeat apt-add-repository without relying on return value
Travis: Repeat apt-add-repository without relying on return value

Recent failures on travis seem to point towards apt-add-repository
not returning a non-null result when it fails. This is basically
an attempt at a workaround for a presumed bug in apt-add-repository.

Example of such a failure:

https://travis-ci.org/BitcoinUnlimited/BitcoinUnlimited/builds/401836627

Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited