Add rc tag to BU version if rc is higher than 0 (#1519)
Add rc tag to BU version if rc is higher than 0 (#1519)

I've tested it via using `make dist` and producing a gitian set of binaries. All the archives and the directories involved properly use the correct version, e.g. `BUcash-1.5.1.0rc2-linux64.tar.gz`
Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited