(maint) Update README stating stdlib 4.x supports Puppet 2.7.x

Without this patch there is a disconnect between the documentation in
the README and our decision to not merge pull requests into the 4.x
series that break compatibility with Puppet 2.7.x

For example:

    @jeffmccune I think the real issue here is that "policy" is out of sync with
    the documentation. The README claims that 4.x does not support puppet 2.7.x,
    yet the "policy" is not to merge patches that break 2.7.x. Due to that I'm sure
    there are a lot of 2.7.x installations out there that have a 4.x version of
    stdlib installed. That's going to cause a rather rude surprise if some future
    version of 4.x stops working where a prior minor release was functioning.

    I'd like to suggest that the documentation be changed to reflect 4.x supporting
    2.7.x and that a new major version bump is made when 2.7.x support can in fact
    be dropped. An alternative solution would be update the README with a note to
    developers about the kinda/sorta/maybe/fishy/quasi support of 2.7.x.

Please also see this discussion:
https://github.com/puppetlabs/puppetlabs-stdlib/pull/176#issuecomment-30251414