Dgraph v0.4.2 release

Improvements

• Switch to tecbot gorocksdb which allows opening stores as read-only
• Build static binaries for Linux and Mac for ease of use
• Added clients in Python and Java to separate repositories
• Replaced glog with net/tracer which does RPC tracing and exposes HTTP interfaces on /debug/requests and /debug/events for easier debugging.

Others

• Setup TravisCI and Code Coverage.
• Also set up Dgraph wiki at wiki.dgraph.io.