refactor(loader): v2.0.0

[joshwiens]

Release branch for Cache Loader v2

• Drops Node 4.x
• Increments Webpack peerDependency "webpack": "^3.0.0 || ^4.0.0"
• Updates babel env target
• Removes cache prefix
• Adds schema enforcment

[michael-ciniawsky]

We still need to support node 4 when webpack >= v3.0.0 is officially supported, if v2.0.0 (#21, #22) is on hold until webpack >= v4.0.0 && node >= v6.0.0 we should release a minor with #24 && #19 meanwhile imho. #21, #22 aren't of any urgency at all

[joshwiens]

Node 4 is being dropped across the entire organization in the next major version of every loader & plugin. What we need to do in regards to NodeJS support has already been decided.

[alexander-akait]

Done in master