Update index.md

Updated installation instructions via composer to use the `composer require` command without specifying a version.
This is the new recommended way of providing this information and it leads to composer picking the proper version constraint (i.e ~1.2), updating the `composer.json` and running install, all in one command.
Merge pull request #163 from rdohms/patch-1

Update index.md
Merge remote-tracking branch 'FOS/master'