go.mod: add Go version directive

Go 1.13 really wants a Go version declaration at the bottom of the file
and will modify it when any go command is run if it isn't present.
Because we'd like our files to be tidy and not show up as modified when
we build or run tests, add a Go version declaration for Go 1.11 (the
oldest we support) at the bottom of the file.

Author: bk2204

go.mod

@@ -5,3 +5,5 @@ require (
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/stretchr/testify v1.2.2
 )
+
+go 1.11