Add ecdsa testing to travis

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.travis.yml b/.travis.yml
index adc6973..44b48b3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,6 +8,7 @@
   - cd sim
   - cargo test
   - cargo test --features sig-rsa
+  - cargo test --features sig-ecdsa
   - cargo test --features overwrite-only
 
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