Upgrade xcconfigs for Xcode 9.3

[mattrubin]

No description provided.

[codecov]

Codecov Report

Merging #172 into develop will decrease coverage by 1.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #172      +/-   ##
===========================================
- Coverage    95.47%   94.38%   -1.09%     
===========================================
  Files            6        6              
  Lines          486      392      -94     
  Branches        32        0      -32     
===========================================
- Hits           464      370      -94     
  Misses          22       22

Impacted Files	Coverage Δ
Sources/Keychain.swift	84.48% <0%> (-3.28%)	⬇️
Sources/Token+URL.swift	96.82% <0%> (-0.58%)	⬇️
Sources/PersistentToken.swift	100% <0%> (ø)	⬆️
Sources/Token.swift	100% <0%> (ø)	⬆️
Sources/Crypto.swift	100% <0%> (ø)	⬆️
Sources/Generator.swift	100% <0%> (ø)	⬆️

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