all: add go version to go.mod

Change-Id: If3becd77d77285bce5e39aa211ea48710b52ef46
Reviewed-on: https://go-review.googlesource.com/c/text/+/192324
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

go.mod

diff --git a/go.mod b/go.mod
index 5eb1e8b..8ff7ecb 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module golang.org/x/text
 
 require golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e
+
+go 1.11