Drop support for PHP 5.2

It's end of life, less than 20% of our user base is on it and being on 5.3 will allow us to use modern PHP features. Anonymous functions FTW!

We should drop it in a minimal way to begin with and then in the future we can look at bringing in some of the improvements that being on 5.3 will make possible.

### Things to change

- [x] Bump the minimum version to 5.3
- [x] Remove the 5.2 unit tests from Travis
- [x] Link to help for 5.2 users