Enable vendoring using GOVENDOR

Resolves - https://discuss.dgraph.io/t/vendoring-dependencies-for-dgraph/177

Pertaining to the discussion mentioned above: GLOCKFILE has been removed,
README has been updated, and vendor/vendor.json has been added. At this
point of time we are only adding dependency information file(vendor.json) and
not the actual dependencies. This commit also helps avoiding
installation of dependencies in $GOPATH if vendor directory is synced
using govendor.