Update Go version in README.md
Update Go version in README.md

Since commit 832bac98 the conf package uses os.LookupEnv, which was added in Go 1.5, thus making gnatsd require Go 1.5+.

This commit updates the README, which still said that Go 1.4+ was required.
glycerine/hnatsd,apcera/gnatsd,wallyqs/gnatsd,apcera/gnatsd,mitghi/gnatsd,nats-io/gnatsd,ericpromislow/gnatsd,mitghi/gnatsd,ericpromislow/gnatsd,glycerine/gnatsd,glycerine/hnatsd,wallyqs/gnatsd,glycerine/gnatsd,nats-io/gnatsd,glycerine/hnatsd