CBLT-61:  use env.var.s instead of hardcoded paths:

{LITESERV_PATH} instead of /Users/couchbase/buildbox/couchbase-lite-ios/build/Release/LiteServ
{SYNCGATE_PATH} instead of /Users/couchbase/buildbox/sync_gateway/bin/sync_gateway

Change-Id: Iad91fe869b7a51ad3d279b7adab8cded6760e1cf