Enable slot0 validation in travis

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.travis.yml b/.travis.yml
index 44b48b3..397c3a5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,6 +10,7 @@
   - cargo test --features sig-rsa
   - cargo test --features sig-ecdsa
   - cargo test --features overwrite-only
+  - cargo test --features validate-slot0
 
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