Add version tags to builds from source.

[danielmai]

When building from source the version tags are missing:

$ $GOPATH/bin/dgraph version

Dgraph version   : 
Commit SHA-1     : 
Commit timestamp : 
Branch           : 

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It'd be valuable to fill out the commit and branch info for source builds. We could create a Makefile that does this, similar to how release.sh sets the version info.