Refactor/networking

[prafsoni]

• Split SwiftNIO and URLSession based networking layer into individual modules ElasticSwiftNetworkingNIO & ElasticSwiftNetworking as respectively.
• add podspec for ElasticSwiftNetworkingNIO
• add travis linux build for swift-5.1
• add Ubuntu 16.04 [swift-5.0.3, swift-5.1] build
• Doesn't use URLSession based networking implementation on Linux.

[codecov[bot]]

Codecov Report

Merging #57 into master will increase coverage by 0.07%. The diff coverage is 25.53%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage    33.9%   33.97%   +0.07%     
==========================================
  Files          48       49       +1     
  Lines        5159     5148      -11     
==========================================
  Hits         1749     1749              
+ Misses       3410     3399      -11

Impacted Files	Coverage Δ
Sources/ElasticSwift/Responses/Response.swift	47.05% <ø> (+8.96%)	:arrow_up:
Sources/ElasticSwiftNetworkingNIO/HTTPClient.swift	81.63% <ø> (ø)
.../ElasticSwiftNetworkingNIO/HTTPClientAdaptor.swift	96.55% <ø> (ø)
...ources/ElasticSwiftNetworking/SessionManager.swift	0% <ø> (ø)	:arrow_up:
...lasticSwiftNetworking/ElasticSwiftNetworking.swift	0% <0%> (-52.64%)	:arrow_down:
...SwiftNetworkingNIO/ElasticSwiftNetworkingNIO.swift	100% <100%> (ø)
Sources/ElasticSwift/ElasticSwift.swift	51.42% <40%> (ø)	:arrow_up:

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