skip all versions that are not lax

I got this:
Invalid version format (non-numeric data) at /Users/ether/.perlbrew/libs/19.9@std/lib/perl5/Dist/Zilla/Plugin/CheckPrereqsIndexed.pm line 54.

...when I had a prereq specification like this: ">= 0.04, != 0.06"

Now, you did have this regexp: `/<>=,\s/`, but I think you meant `/[<>=,\s]/` ;)  But just checking for whether `version->parse` will blow up is a little more safe.

(Of course, the *real* fix should be to use CPAN::Meta::Check to parse the version spec against what's installed, but that's a larger change and would benefit by a bit more code reorg. Another day, I think!)