Add extra multi-image testing to Travis-CI

Enable testing of sig-rsa + multi-image + validation on Travis.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.travis.yml b/.travis.yml
index 089bfa9..e38388e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -25,7 +25,7 @@
     - os: linux
       env: MULTI_FEATURES="sig-rsa overwrite-only,sig-ecdsa overwrite-only,multiimage overwrite-only" TEST=sim
     - os: linux
-      env: MULTI_FEATURES="sig-rsa validate-primary-slot,sig-ecdsa validate-primary-slot" TEST=sim
+      env: MULTI_FEATURES="sig-rsa validate-primary-slot,sig-ecdsa validate-primary-slot,sig-rsa multiimage validate-primary-slot" TEST=sim
     - os: linux
       env: MULTI_FEATURES="enc-kw overwrite-only,enc-rsa overwrite-only" TEST=sim
     - os: linux