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 <[email protected]>

Author: utzig

.travis.yml

@@ -43,7 +43,7 @@ matrix:
       language: go
       env: TEST=mynewt
       go:
-        - "1.11"
+        - "1.12"
 
 before_install:
   - |