fix how to extract perl version from v-string

[skaji]

fix #198

As in #198, if a pm file contains use v5.20;, then minilla produces META.json containing

   "prereqs" : {
      ...
      "runtime" : {
         "requires" : {
            "perl" : "5.20"
         }
      },

which means this distribution requires perl 5.200_000.

This PR fixes this issue by extracting perl version together with leading "v".

[syohex]

Looks good

[skaji]

@tokuhirom @syohex Thank you for your review!