add Travis build status badge to README

[mykmelez]

Once we start testing the commands (#22, #34, #35, #36), we should add a Travis build status badge to the README.

[mykmelez]

There are a ton of other badges we might add, such as those implemented by the badges module and those supported by http://shields.io/. But the Travis one is the only one that I know would be useful to advertise.

[marco-c]

I've found the dependencies and devDependencies badges to be useful too (for example I once chose a library over another because the green badges meant the library was actively developed).

A code coverage badge would be useful too to advertise the level of quality of the library, but we currently have 0% coverage 😄