Use Object.defineProperty instead of __defineGetter__

__defineGetter__ is not standardized and not supported in IE. It thus breaks Travis completely in IE. This not only fixes https://github.com/travis-ci/travis-ci/issues/1001 but also ensures that the JavaScript code is standards-compliant.
Merge pull request #1 from lanthaler/use-defineProperty-not__defineGetter__

Use Object.defineProperty instead of __defineGetter__