ci: enable enc-ec256 test

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.travis.yml b/.travis.yml
index 8d4530f..a7587b7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -21,7 +21,7 @@
     - os: linux
       env: SINGLE_FEATURES="none sig-rsa sig-rsa3072 overwrite-only validate-primary-slot swap-move" TEST=sim
     - os: linux
-      env: SINGLE_FEATURES="enc-rsa" TEST=sim
+      env: SINGLE_FEATURES="enc-rsa enc-ec256" TEST=sim
 
     # Values defined in $MULTI_FEATURES consist of any number of features
     # to be enabled at the same time. The list of multi-values should be