Add support for travis-ci to deploy to rubygems #18

Each gem's schema_dev.yml file should allow an optional field:

travis_rubygems_key: O6ppogYENdh7TprJbbW5OkKMEUAdZioZOWUEdm84+B/6hzYpPbvDsqrX+NbOk8f94r7pllxwribJypLN3VmIj5DDKKBMWW4YSfUQFpN6S0oVh2zlu4cINeEs8IhxP2I8rHesr0T6LN0j7CskImO5E1sKwrTL1E8EKLuUASK658I=

If present, schema_dev would include this magic in the .travis.yml file, telling Travis to deploy to rubygems whenever a tagged version passes the tests.

deploy:
  provider: rubygems
  api_key:
    secure: O6ppogYENdh7TprJbbW5OkKMEUAdZioZOWUEdm84+B/6hzYpPbvDsqrX+NbOk8f94r7pllxwribJypLN3VmIj5DDKKBMWW4YSfUQFpN6S0oVh2zlu4cINeEs8IhxP2I8rHesr0T6LN0j7CskImO5E1sKwrTL1E8EKLuUASK658I=
  gem: schema_plus_core
  on:
    tags: true
    repo: SchemaPlus/schema_plus_core

(Of course using the current gem's name for the repo field.) See Travis-CI RubyGems Deployment.

Probably the easiest way to generate the key would be for a rubygems owner of the gem to run travis setup rubygems which would add all the above to the .travis.yml. Then copy and paste the key to the schema_dev.yml file; then run schema_dev freshen