Update to work with latest Babel 7 beta

[dfreeman]

This should fix #226 and get things in sync with the most recent Babel 7 beta. A few changes of note:

• the various @babel packages no longer support node 4, so I updated the engines field and Travis/Appveyor config here to match what I believe is the standard support matrix for new addons today
• @babel/preset-env is now way pickier about what config it receives, so we need to whitelist the keys we pass in
• Expose all filenames as absolute paths, rather than relative. babel/babel#8044 means all module paths are now absolute, so our config for Babel itself and amd-name-resolver needed to be tweaked to account for that
• the latest babel-plugin-debug-macros has a new preferred config format

/cc @rwjblue

[rwjblue]

Awesome, thank you!