v3.1.1 release

[springmeyer]

Quick release to get out node 5.x support (with binaries)

TODO:

• Write a CONTRIBUTING.md doc so that @Mithgol @ErisDS could follow it in the future to push out a release (will pull from https://github.com/mapnik/node-mapnik/blob/master/CONTRIBUTING.md)
• Bump the version and get latest deps - 77d51d5
• Ensure binaries built okay
• Ensure changelog is up to date
• Tag and publish

[springmeyer]

@Mithgol @lvivier - I see you've added a few things to master, can you update the changelog with the key details?

[springmeyer]

v3.1.1 is now tagged and published and supports Node 5.x (with binaries).

[lvivier]

👍