As this repo it is no longer in external/source directory, the deploy
targets need to know where the Framework repository is. This defaults to
../metasploit-framework, but can be changed via -D deploy.path= command
line switch.
Apparently, getNetworkPrefixLength can return -1, which confuses the Ruby
side. Therefore fall back to guessing the prefix in this case, as we do it
for Java <= 1.6.
