I'm going to go against the grain here and say people are massively over-speccing the kind of hardware you actually need. I'm a web dev contractor and still use a 2011 1.7ghz air with 4gb ram as my daily machine. While it's getting a bit long in the tooth and probably due an upgrade, it's still great for running Chrome, PHPStorm, and a VM or two.
Even the base model 13" Macbook Air has more than enough power to use as a dev machine so the only thing that you should base the decision on is which is more important to you - portability or the retina screen. Once you've made the decision by all means spec it up as much as you can afford for future proofing, but understand that you don't actually *need* it.
The biggest improvement you can make to a dev environment is a second screen, so if you've already got that sorted then you're good to go (the Air will need an adaptor if you've not got displayport input).
Even the base model 13" Macbook Air has more than enough power to use as a dev machine so the only thing that you should base the decision on is which is more important to you - portability or the retina screen. Once you've made the decision by all means spec it up as much as you can afford for future proofing, but understand that you don't actually *need* it.
The biggest improvement you can make to a dev environment is a second screen, so if you've already got that sorted then you're good to go (the Air will need an adaptor if you've not got displayport input).