travis: add sig-ecdsa + enc-ec256 to sim

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.travis.yml b/.travis.yml
index a831ccf..7eacbdc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -35,7 +35,7 @@
     - os: linux
       env: MULTI_FEATURES="sig-ecdsa enc-kw validate-primary-slot" TEST=sim
     - os: linux
-      env: MULTI_FEATURES="sig-rsa validate-primary-slot overwrite-only" TEST=sim
+      env: MULTI_FEATURES="sig-rsa validate-primary-slot overwrite-only,sig-ecdsa enc-ec256 validate-primary-slot" TEST=sim
 
     - os: linux
       language: go