Add constraint for bradleyfalzon/ghinstallation (#29)

As of 1.0.0, this library includes a breaking API change and appears to
require that we adopt modules to be compatible with the go-github
imports.

Author: bluekeyes

Gopkg.lock

@@ -3,6 +3,10 @@ ignored = [
   "github.com/golang/protobuf/proto"
 ]
 
+[[constraint]]
+name = "github.com/bradleyfalzon/ghinstallation"
+version = "<1.0.0"
+
 [prune]
   non-go = true
   go-tests = true