v4.0.0

• [BREAKING] gitDescribe cannot be called synchronously anymore; use gitDescribeSync if you need a synchronous API.
• [BREAKING] Semver dependency is now optional.
• [BREAKING] Dropped support for Node.js v0.12 and below.
• Added option dirtyMark for customizing git-describe's dirty string. [#4]
• gitDescribe now has a dual promise/callback asynchronous API.
• Greatly improved test coverage.