Wrong message when trying to use PHP-CGI if not available

[stof]

when trying to use CGI on PHP 5.3 on travis (which does not have CGI on 5.3), the following message is displayed:

rbenv: php-cgi: command not found

The `php-cgi' command exists in these Ruby versions:
5.2
5.2.17
5.3.3
5.4
5.4.13
5.5
5.5.0beta1

This message is useful, but it would be better if it was talking about PHP versions rather than Ruby versions.
Originally reported as travis-ci/travis-ci#1078

[CHH]

That's not an easy one. One would need to sed this in rbenv. But may be not so a big issue, as we already sed a bunch of other things anyway.