Update mynewt travis test to use go 1.12

`newt` was recently updated to build with go module and requires go
1.12.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.travis.yml b/.travis.yml
index 42f6d40..3bb0751 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -43,7 +43,7 @@
       language: go
       env: TEST=mynewt
       go:
-        - "1.11"
+        - "1.12"
 
 before_install:
   - |