Add empty script

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.travis.yml b/.travis.yml
index badc73b..79af9e8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -18,3 +18,6 @@
     notification_email: utzig@apache.org
     build_command: "make -f Makefile.coverity"
     branch_pattern: "coverity_scan"
+
+# do nothing here because coverity already runs the "build_command" above
+script: ""