Bump docs to suggest 5.3 is used

A while back when I was on the team I suggested we moved from 5.1 to 5.2. This made sense two or three years ago when this branch was meant to be "out soon" but time has moved on a little bit. 

When people first started grumping about 5.2 being used I explained that I didn't think it was so bad for various reasons. I know that people use CodeIgniter on versions of PHP that are WAY out of date, and I know that even 3 years ago moving from 5.1 to 5.2 pissed people off. 

What has made me spin on a dime here is that the .travis.yml does not actually test against 5.2 at all, and of course it can't because vfStream has a [5.3 dependency](https://github.com/mikey179/vfsStream/blob/fc0fe8f4d0b527254a2dc45f0c265567c881d07e/composer.json#L7). 

On top of that, PHPUnit probably needs 5.3 too depending on the version travis is supplying you with.

SO! 

If you aren't testing against 5.2, you really really cannot release with docs saying that you support 5.2. CodeIgniter was always a cowboy framework with zero tests and that is turning around. In the past were dangerous untested releases done without warning. It was a bit of a nightmare, and I had to run back from the pub one day to patch and retag a version that somebody at EllisLabs had released without warning which had some bugs on one version of PHP. Without 5.2 in the `.travis.yml` version matrix once again I'd say you absolutely cannot say you support 5.2.

Instead of trying to find a way to test on 5.2, I'd urge you just to pop up the requirements to 5.3.

This may well still work on 5.2 and the only place it will fail is if a developer tries to run `composer update`, but that is only for testing, and the tests wont work anyway.

I'm not going crazy here. I think PHP 5.4 would be nice, but I know that these things need to be done slowly. If you lot want to go further than carry on, but everyone else should try and avoid complaining that the level of progress is not enough.
This is the least find-and-replace bit I did. It's an important message, to remind people to upgrade to later versions. Most wont bother I know, but many will.