Fix: properly convert `NSString?` to `String?`

[tconkling]

This fixes the following warning:

RunResults.swift:39:82: Conditional downcast from 'NSString?' to 'String' is a bridging conversion; did you mean to use 'as'?

[codecov-io]

Codecov Report

Merging #34 into master will not change coverage. The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #34   +/-   ##
=======================================
  Coverage   97.01%   97.01%           
=======================================
  Files          39       39           
  Lines        1543     1543           
=======================================
  Hits         1497     1497           
  Misses         46       46

Impacted Files	Coverage Δ
Source/RunResults.swift	87.5% <0%> (ø)	:arrow_up:

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[klaas]

I bet if this gets pulled into master, Swiftline will reach 1000 stars on github!!

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