Version 0.15.0: Add averageResult and canReach functions

This is the last version to support Swift 4.0-4.0.3 and Swift 4.1-4.1.3

Here's what's new:

• CODE_OF_CONDUCT.md
• CONTRIBUTING.md
• Rollable
  • averageResult by @Taufi(https://github.com/Taufi)
  • canReach(_:_:)
• Die.doubleAverageResult by @Taufi(https://github.com/Taufi)
• RollComparison: denotes the comparison to use in `canReach(::)
• Fixed Linux random on versions of Swift before 4.2
• Fixed in Dice
  • Include modifiers in Dice addition functions
  • Properly multiply Dies in Dice multiplication functions.

See changelog
See docs