Updates travis to use swift 4.0 release

[sandmman]

No description provided.

[codecov-io]

Codecov Report

Merging #38 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   75.84%   75.84%           
=======================================
  Files           1        1           
  Lines         828      828           
=======================================
  Hits          628      628           
  Misses        200      200

Flag	Coverage Δ
#SwiftyJSON	75.84% <ø> (ø)	⬆️

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