go.mod: set go version

Change-Id: I45f1816a359ad6fab548338fd3a6b01dda32d01f
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/203838
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: Bryan C. Mills

go.mod

@@ -1,5 +1,7 @@
 module golang.org/x/crypto
 
+go 1.11
+
 require (
 	golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3
 	golang.org/x/sys v0.0.0-20190412213103-97732733099d