Jest 24

Created by: leontastic

Handles the following concerns:

• Update Jest to version 24
• Use setupFilesAfterEnv in Jest config instead of deprecated setupTestFrameworkScriptFile option
• Add setupFilesAfterEnv to list of overridable Jest config options
• Updated documentation to reflect new functionality described above
• Clean up createJestConfig

I'd like to note specifically that the logic associated with setupTests has been preserved, but it can now be overridden with Jest's shiny new setupFilesAfterEnv config option ✨