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"
```
Merge pull request #121 from philips/alternative-go-version-check

feat(go_version): check go version at build time