cache folder depth fix and updated dependencies

From the CHANGELOG:

• fixed: too many folder levels for docker to cope with (see Issue 138)
• fixed: Usages of new Buffer(…) have been changed to Buffer.from(…) – see DEP0005 for more info
• bumped node engine version in package.json to support more versions – npm test succeed for v4.x up to edge v12 atm
• bumped dependencies
• moved nodeunit dep into devDependencies - it's deprecated and using a specific sshpk lib version was necessary to reach npm audit result of 0 known vulnerabilities

Beware: The change of the internal cache folder structure means, that your cache is invalidated upon deployment of this version. This shouldn't affect your apps too much as long as you took the hint of running converjon behind a caching reverse proxy. Keep it in mind though when switching to this version as your cache folder size may double.