Update CI, vulnerability checking, and dependencies.

[adam-moss]

Add snyk for vulnerability checking.

Update all dependencies to the latest versions to resolve identified vulnerabilities.

Update tests broken by the dependency update of should.

Update CI to include all long-term support and major releases of node, and iojs.

[coveralls]

Coverage remained the same at 94.755% when pulling 1c697c6 on adam-moss:update-dependencies into 57db719 on nickmerwin:master.

[pwfisher]

The update of the request dependency will resolve broken builds due to the pinning of the specific version, 2.69.0. See request/request#2040. Please update this dependency.

[coveralls]

Coverage remained the same at 94.755% when pulling a87354b on adam-moss:update-dependencies into 55ebe27 on nickmerwin:master.

[adam-moss]

Dependencies updated to latest releases.