Make Vaccine compatible with Swift 4.0

[zenangst]

Adds #if statement to handle Swift 4.0

[codecov-io]

Codecov Report

Merging #50 into master will increase coverage by 1.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage    27.2%   28.46%   +1.26%     
==========================================
  Files          16       16              
  Lines         397      404       +7     
==========================================
+ Hits          108      115       +7     
  Misses        289      289

Impacted Files	Coverage Δ
Source/iOS+tvOS/UIViewController+Extensions.swift	71.17% <100%> (+1.94%)	⬆️

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