.travis.yml: include go1.12 (#932)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
diff --git a/.travis.yml b/.travis.yml
index fd650b2..b7c4c97 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,6 +4,7 @@
 - 1.9.x
 - 1.10.x
 - 1.11.x
+- 1.12.x
 - 1.x
 
 install: