Version regex is too restrictive

The version regex present in `res/schema.json` invalidates versions that follow semantic versioning if they do not include a `-` character between the version and any stability qualifiers.

While semver.org does specify that any stability or metadata information MAY presented following such a character, in practice, most libraries do not use the character. (E.g. PHP itself, PHPUnit, most pecl extensions, etc.). In fact, the PHP function `version_compare` is able to use either with or without a dash as well.

Essentially, I'd like to see each of `1.0.0-beta1`, `1.0.0-b1`, `1.0.0beta1`, and `1.0.0b1` validate under the schema. I'll prepare a PR shortly.