use Swift 4

[omochi]

My environment

macOS High Sierra 10.13.4
Xcode9.3

Problem

I just cloned master and got compile error below.

/Users/omochi/github/omochi/RandomKit/Sources/RandomKit/Extensions/Swift/Collection+RandomKit.swift:100:17: Invalid redeclaration of 'uncheckedRandom(in:using:)'
/Users/omochi/github/omochi/RandomKit/Sources/RandomKit/Extensions/Swift/Collection+RandomKit.swift:75:17: 'uncheckedRandom(in:using:)' previously declared here
/Users/omochi/github/omochi/RandomKit/Sources/RandomKit/Extensions/Swift/Collection+RandomKit.swift:106:73: Type 'Collection.IndexDistance' (aka 'Int') in conformance requirement does not refer to a generic parameter or associated type

The lines with error is in macro section for Swift less than 4.1.
And a xcodeproj set swift version to 3.

Solution

This PR changes Swift version from 3 to 4 and avoid this error.