Fix version scheme to be semver compatible

[thet]

yarn refused to install patternslib with a semantic versioning incompatible version scheme.

Try:

> const semver = require('semver');
> semver.valid('4.0.0')
'4.0.0'
> semver.valid('4.0.0dev')
null
> semver.valid('4.0.0.dev')
null
> semver.valid('4.0.0-dev')
'4.0.0-dev'