use version::is_lax for string parsing

(Continued from #2) 

I've found some TODO comments in code, and because version 0.81 was out (some time ago), I used `version::is_lax` for string parsing as you had intended.

(Copied [this comment](https://github.com/dagolden/Version-Next/pull/2#issuecomment-36899914).)

Using `version::is_lax` handles some invalid strings like `1_09_1` or `1._0_9` while your regexp returns true, and later Version::Next warns:

    Argument "1._0_9" isn't numeric in addition (+) at /usr/local/perl514/lib/site_perl/5.14.2/Version/Next.pm line 69

when `version::is_lax('1_09_1')` returns false.

It seems `version::is_strict` does not handle dev versions, so `version::is_strict('1.01_01')` returns false.