Add node v8 and v10 support

[springmeyer]

Currently we are only building for node v6. Let's add v8 and v10 support in the next dev release to ensure that vtcomposite is ready to be immediately tested against these versions in downstream APIs.

To add node v8 and v10 support:

• Add new jobs to the .travis.yml. 2 jobs for both v8 and v10 on OS X and 4 jobs for v8 and v10 on linux (but both release and debug). Learn about a debug build here. Note: we want debug builds to be available in case production crashes on linux happen and we need them to diagnose a crash. Example PR to follow: https://github.com/mapbox/node-cpp-skel/pull/144/files
• Upgrade to latest Nan and node-pre-gyp. The latest releases of these tools have support for node v10.
• Make code changes to fix the deprecation warnings related to the use of MakeCallback and Call. Example PR to follow: https://github.com/mapbox/node-cpp-skel/pull/127/files
• Make sure all tests are passing on travis for all the new node versions you've added

Refs mapbox/node-cpp-skel#125