Continuous Integration (Linux)

Hi :)

This could be the first step towards strong test coverage.

All code is taken from the latest Bundler templates, with minor edits. I actually ran `bundle gem rmagick --test=minitest --ext` and copied files over from the created directory. So I think this represents current community best practices.

You can see build logs here for now:

https://travis-ci.org/vassilevsky/rmagick

After merge, you will need to sign in to Travis yourself and enable building of **gemhome/rmagick**. After that, all pull requests will be checked (built).

The next step would be to actually get the tests passing.

Hope this helps.