Update travis to use go 1.13
diff --git a/.travis.yml b/.travis.yml
index 0e43d65..658f63c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -26,7 +26,7 @@
       - linux-libc-dev:i386
 
 go:
-  - "1.12"
+  - "1.13"
 
 git:
   depth: false