Fix String warnings for Xcode 9.1

[ziogaschr]

No description provided.

[codecov]

Codecov Report

Merging #149 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           develop    #149   +/-   ##
=======================================
  Coverage     94.7%   94.7%           
=======================================
  Files            6       6           
  Lines          378     378           
=======================================
  Hits           358     358           
  Misses          20      20

Impacted Files	Coverage Δ
Sources/Generator.swift	100% <100%> (ø)	⬆️

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[mattrubin]

Thanks, @ziogaschr!