feat(go_version): check go version at build time

```
$ ./build
can't load package: package github.com/coreos/etcd:
src/github.com/coreos/etcd/go_version.go:3:1: expected 'package', found
'STRING' "etcd requires go 1.1 or greater to build"
```

go_version.go

@@ -0,0 +1,3 @@
+// +build !go1.1
+
+"etcd requires go 1.1 or greater to build"