Switch to Go 1.11.5

An earlier commit incorrectly updated some versions in go.mod without also
updating the vendor tree, so this also rolls those back to where they used
to be so that we can roll them forward carefully and make sure the tests
actually pass. (If we just accept these new versions as specified the
tests do not pass, so some work is required to fix those regressions.)

.go-version

@@ -1 +1 @@
-1.11.4
+1.11.5