simplifying travis deploys

[jackfrancis]

Simplifies deploys to prod/staging the following:

• merges to "staging" deploy to the staging environment
• merges to "master" deploy to the production environment

using IMAGE_PREFIX=deisci (provided by .travis.yml) for all deploys

[arschles]

@jackfrancis how does this simplify deploys?

Also, it seems like this PR changes two different things at once. Can you explain the changes to the .travis.yml file?