Add support for Travis CI builds

[harrisonhjones]

This code change adds support (a config file) for Travis CI builds. It also adds a build badge to the README. Example of it building my fork.

To get this to work you will also need to:

1. Head over to Travis CI and sign-in with your GitHub account
2. Once signed in you should get a list of your repos. Locate go-alexa and select it.
3. Locate the blue green "Activate repository" button. Press it and follow any additional steps to get it setup.
4. Travis CI will automatically start building new pull requests and commits (this can be customized). Once this PR is merged all new code will be built (using go test) against go 1.8, 1.9, 1.10, 1.x, and "tip". The build badge on the README will reflect the current state of building master.

[rking788]

Looks good.

[harrisonhjones]

A last-minute thought: some projects have their badges next to the header in their README. Should we do the same here? I was thinking we could also add a badge link to our godoc and to the go report card but that should be in a separate PR.

[mikeflynn]

This is awesome. I fixed the conflict in the README and setup the account, so I'm merging this in.

(The go tip job is taking a while to complete, but for this PR I think it's OK to merge without it!)