Use quotes for PHP version numbers in README

[osma]

The PHP version numbers in the README did not use quotes. This is fine for e.g. 7.1, 7.2, 7.3 and 7.4 but not 8.0, which is interpreted as 8 by YAML and currently resolves to 8.1.1. See this comment by @ngrie for an explanation.

Fixes #73

(I suppose a similar fix should be made also for other php-actions modules which use the same mechanism, at least php-actions/phpunit)

[g105b]

Thank you @osma - I didn't know that was how YAML numbers worked. In fact, I find all YAML config confusing, so I'm glad you made this change as it will help a lot of future developers out.

Thanks again.

[osma]

I find YAML confusing too, and I didn't know about the number parsing either until I found out the hard way - thanks again @ngrie for clarifying this!

Thanks for merging!