add support for the nodejs6.10 runtime

[lusentis]

docker-lambda has just added support for this runtime.

• [x] update babel-preset-env target in https://github.com/dawson-org/babel-preset-dawson
• [x] update babel-preset-dawson version
• [x] set as default behavior in template factories
• [x] update docker-lambda
• [x] update documentation and remove support for nodejs4.3

It makes no sense as of now to support both runtimes. In the future, we can easily support multiple runtime versions, if that's needed. The current dawson runtime logic is named "javascript-latest", so it will be updated to match nodejs6.10. If anyone's experiencing issues related to this change, please re-open & comment below.

[lusentis]

Released as 0.24.0 🎉