Add ECDSA+KW testing to the CI

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.travis.yml b/.travis.yml
index a18bfb8..87ba9f2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -29,6 +29,8 @@
       env: MULTI_FEATURES="sig-rsa enc-rsa validate-slot0"
     - os: linux
       env: MULTI_FEATURES="sig-rsa enc-kw validate-slot0 bootstrap"
+    - os: linux
+      env: MULTI_FEATURES="sig-ecdsa enc-kw validate-slot0"
 
     # FIXME: this test actually fails and must be fixed
     #- os: linux