Updated to Swift 4.0

[Davidde94]

The package as a whole has been upgraded to have full support for Swift 4 (with all errors/warnings handled).

Description

Updated code to Swift 4 format (e.g. not using String.characters anymore)

Motivation and Context

This was a requirement in order for https://github.com/IBM-Swift/Kitura-Session-Redis/pull/34 to be merged

How Has This Been Tested?

The pre-existing test suite was run, and all tests passed.

Checklist:

• [x] I have submitted a CLA form
• [x] If applicable, I have updated the documentation accordingly.
• [x] If applicable, I have added tests to cover my changes.

[codecov-io]

Codecov Report

Merging #61 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   80.93%   80.94%   +0.01%     
==========================================
  Files          17       17              
  Lines        2381     2383       +2     
==========================================
+ Hits         1927     1929       +2     
  Misses        454      454

Flag	Coverage Δ
#SwiftRedis	80.94% <100%> (+0.01%)	:arrow_up:

Impacted Files	Coverage Δ
Sources/SwiftRedis/RedisInfo.swift	91.89% <100%> (+0.46%)	:arrow_up:

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