Migrate to Swift 4.0.2

[DunnCoding]

Removes backwards compatibility, full migration to Swift 4.0.2

[tunniclm]

@sandmman As this will break compatibility with Swift 3, I assume we want to bump the major (or at least the minor) when we tag this?

[codecov-io]

Codecov Report

Merging #70 into master will decrease coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
- Coverage    49.6%   49.47%   -0.13%     
==========================================
  Files          12       12              
  Lines        1633     1633              
  Branches       71       71              
==========================================
- Hits          810      808       -2     
  Misses        799      799              
- Partials       24       26       +2

Flag	Coverage Δ
#CouchDB	49.47% <100%> (-0.13%)	⬇️
#CouchDBSample	49.47% <100%> (-0.13%)	⬇️

Impacted Files	Coverage Δ
Sources/CouchDB/Database.swift	48.45% <100%> (-0.29%)	⬇️
Sources/CouchDB/ConnectionProperties.swift	39.28% <0%> (-3.58%)	⬇️

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[sandmman]

@tunniclm We've been trying to stick with updating the major for breaking changes and minors for new api features. Since we lose Swift 3 support as you mentioned, this will go to 2.0.0.

[sandmman]

Looks good!