Run RSA-3072 feature on travis-ci

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.travis.yml b/.travis.yml
index 042970f..4ca97f7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,7 +14,7 @@
     - os: linux
       env: SINGLE_FEATURES="sig-ecdsa enc-kw bootstrap" TEST=sim
     - os: linux
-      env: SINGLE_FEATURES="none sig-rsa overwrite-only validate-primary-slot" TEST=sim
+      env: SINGLE_FEATURES="none sig-rsa sig-rsa3072 overwrite-only validate-primary-slot" TEST=sim
     - os: linux
       env: SINGLE_FEATURES="enc-rsa" TEST=sim