Add Travis CI config to build Marlin firmware.

This CL adds a [Travis CI](https://travis-ci.org/) config to allow automatic builds on every commit and send an email, if the build is failed. It also adds a build status banner, like below:

[![Build Status](https://travis-ci.org/krasin/Marlin.svg)](https://travis-ci.org/krasin/Marlin)

The only difference is that the banner above points to my fork, and the banner proposed by this CL points to the not-yet-existing profile for ErikZalm.

This CL also opens a door for running automated tests on each commit.

To enable Travis CI on github.com/ErikZalm/Marlin, the following quick setup is needed:

1. Go to https://travis-ci.org/ and click "Sign in with GitHub" button in the upper right corner.
2. Grant the requested permissions (it will not ask for code access, just commit hooks/list emails/access project activity)
3. Go to https://travis-ci.org/profile and enable builds on ErikZalm/Marlin repository
4. Merge this CL and enjoy the reduced risk to break the build and not to get noticed.

Please, let me know, if this CL needs more clarification. I would be happy to address any concerns.