chore(docs): Point INSTALL.txt to learn.elgg.org

Don't want to have to keep these two versions of instructions in sync.
feature(requirements): PHP 5.3.3+ is now required

Symfony needs 5.3.3 and we plan on using their components heavily.

Closes #6165
Merge pull request #6926 from ewinslow/php-53-b6165

Require PHP 5.3.3+ to run Elgg