Cross platform target + Travis CI

[lightsprint09]

• Removes all platform specific targets
• Creates a single target which supports iOS, macOS, tvOS, watchOS
• Adds Travis CI configuration
• • Travis currently runs at https://travis-ci.org/lightsprint09/JSONCodable
• • You have to create your own Travis CI instance when this is merged

Read this and this for more information on multiplatform schemes

[Nadohs]

sorry this took so long, thanks for the PR

[matthewcheok]

This looks good! Thanks for submitting this @lightsprint09

[lightsprint09]

Keep in mind that you have to setup Travis CI for this repo now.