Ruby and parts of PHP are the quick wins. The real question when it comes to backend jobs is what is going to happen in the JS backend vs. Java. And that is at the moment hard to predict. The Java eco system is showing some weakness (Oracle stewardship, Eclipse stumbling, IBM embracing Swift) but on the other hand it is massive.
JS is not in a place yet where it could take on Java. But it is on the way to become an alternative that could eat away at Java use cases. JS6, TypeScript and the consolidation around NPM packages are significant improvements. Node control is too concentrated for a long term healthy ecosystem and lacks alternatives but if such would emerge then all bets are off. The primary back-end stack which is at the moment compiled Java byte-code could in the long run split into compiled whatever code and scripted Typescript code.
Caveat: Java investments stay subdued. JS investments don't level off.