Add go 1.11 to go.mod.

Go 1.12 really wants to record a version in go.mod if there is no
version in there. 1.12 is not yet released, so stick 1.11 in there for
now. We'll bump it to 1.12 and so on as we update our minimum versions.

Change-Id: I79ac85837149ab7cadd2f23acd8ab2d207a1a355
Reviewed-on: https://boringssl-review.googlesource.com/c/34924
Reviewed-by: Adam Langley <agl@google.com>

go.mod

@@ -1 +1,3 @@
 module boringssl.googlesource.com/boringssl
+
+go 1.11