Add a condition for `deploy.on` in .travis.yml
Add a condition for `deploy.on` in .travis.yml

This might be required since there is SPM job too.

https://docs.travis-ci.com/user/deployment/#Conditional-Releases-with-on%3A

Carthage/Commandant,Carthage/Commandant,jpsim/Commandant,ikesyo/Commandant