Remove support for Rails 4.2

[luke-hill]

In general: Remove rails 4.2 support from gem

Summary

Remove rails 4.2 from gem support. Requires 3 things.

• rails dropping support for 4.2 Or a period of around 3-6months from below PR
• Extension to ruby support (Currently being worked on in #466)
• Code cleaning around conditionals that support 4.2 (I'm not sure on how extensive this is, but I know it exists).

Once this work has been done, we would then look to "soon" after remove 5.0 support.

Expected Behavior

5.0, 5.1, 5.2, 6.0 all supported

Current Behavior

4.2, 5.0, 5.1, 5.2, 6.0 all supported

Possible Solution

Fix stuff

Steps to Reproduce (for bugs)

Each PR to be done separate, culmination in a release of 2.1

Context & Motivation

Tidy up of old codebase

Your Environment

N/A