Mention correct Go version in README

The new `for-range` syntax [used here](https://github.com/golang/gddo/blob/ff26967b097e6981d2fb66bba2e834e6c4794d92/gosrc/client.go#L118) requires Go 1.4, as mentioned in the [beta release notes](http://tip.golang.org/doc/go1.4#forrange)