Update build.md
Update build.md

Step #7 is needed before step #3 or the later would fail. I suggest moving step #7 to prerequisites chapter since that's what it seems to be.
metabit/bitsquare,ManfredKarrer/exchange,sakazz/exchange,bisq-network/exchange,sakazz/exchange,haiqu/bitsquare,bisq-network/exchange,bitsquare/bitsquare,ManfredKarrer/exchange,metabit/bitsquare,haiqu/bitsquare,bitsquare/bitsquare