Update Bluebird dependency

[ErisDS]

Ghost currently depends on bluebird v2.10.2. Bluebird has progressed on to v3.0.6, which has a number of fixes, improvements and deprecations.

See What's new in 3.0 for a summary of the deprecations and breaking changes.

As part of the upgrade of the dependency, the codebase needs to be combed for breaking changes and deprecations, and these should also be updated to use the newer tools or features.

Note we now use a convention for the title line in a commit message changing a dependency:

deps: [email protected]

closes #6361

- Summary of 
- things that were 
- changed