Claim to require Golang 1.2+ in README.md

in https://github.com/google/cayley/blob/master/make.sh#L28
```
go get -u -t github.com/smartystreets/goconvey
```
-t option is added in Golang 1.2 http://golang.org/doc/go1.2#gocmd

Considering that old Ubuntu and RHEL may still use go1.1 or go1.0, we'd better make it clear that we requires Goland 1.2 here