Increase the minimum engine version

[ricellis]

The current minimum engine version for Nano is Node 0.12. That version is EOL and no longer supported, the oldest LTS stream is the 4.x, although that is only going to be maintained until April 2018 [1].

It also appears that #45 stopped testing Nano on older versions and made 4.x the minimum tested version.

As a result I think it makes sense to increase the minimum engine version to at least 4.

Context

As seen by #62 some of Nano's dependencies (e.g. requests) already specify an engine >=4 and an in-range update of that dependency caused a break for people running nodejs-cloudant and/or Nano on Node 0.12. The changes in #62 will prevent that break, but will also stop further minor version updates of request which may well be needed for vulnerability fixes etc in future (there have been some in Hawk in the past) so I think the change to pin request version can only be a stop-gap.

Expected Behavior

The minimum engine version of Nano should be equal to the greatest minimum engine version of any of the dependencies and should match the oldest tested version.

Current Behavior

The engine version is an EOL, un-maintained version of Node.js (0.12).

Possible Solution

• Update the engine version to >=4, or maybe even >=6 since that will be the oldest maintained LTS in April 2018.
• Re-enable minor version updates of the request dependency.
• I'm not 100% clear from the NPM documentation, but it might be worth considering adding the config: {engine-strict: true} flag to the package.json to by default prevent installing on unsupported engine versions.

[janl]

Would take a PR for engine: 6+

[ricellis]

Done in #87