Specify the Go version in CONTRIBUTING [GH-424]

CONTRIBUTING.md

@@ -47,7 +47,9 @@ it raises the chances we can quickly merge or address your contributions.
 If you have never worked with Go before, you will have to complete the
 following steps in order to be able to compile and test Packer.
 
-1. Install Go. On a Mac, you can `brew install go`.
+1. Install Go. On a Mac, you can `brew install go`. Make sure the Go
+   version is at least Go 1.1. Packer will not work with anything less than
+   Go 1.1.
 
 2. Set and export the `GOPATH` environment variable. For example, you can
    add `export GOPATH=$HOME/Documents/golang` to your `.bash_profile`.