Add separate lint stage to .travis.yml

[MaxDesiatov]

This prevents linting stage from running multiple times in all parallel jobs. We're mostly going to develop with the latest version of Xcode, so we only care about linter and formatter warnings with that version. Thus, the linter stage runs first with the latest version, while actual tests run in parallel jobs of the next "test" stage.

[codecov]

Codecov Report

Merging #87 into master will increase coverage by 1.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #87     +/-   ##
=========================================
+ Coverage   76.93%   78.04%   +1.1%     
=========================================
  Files          37       37             
  Lines        1812     2022    +210     
=========================================
+ Hits         1394     1578    +184     
- Misses        418      444     +26

Impacted Files	Coverage Δ
Sources/XMLCoder/Auxiliaries/XMLStackParser.swift	93.54% <100%> (+0.36%)	⬆️
...rces/XMLCoder/Decoder/DecodingErrorExtension.swift	65.21% <0%> (-34.79%)	⬇️
Sources/XMLCoder/Auxiliaries/XMLCoderElement.swift	97.67% <0%> (-0.04%)	⬇️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	83.98% <0%> (+0.26%)	⬆️
Sources/XMLCoder/Encoder/XMLEncoder.swift	80% <0%> (+0.79%)	⬆️
...s/XMLCoder/Encoder/XMLKeyedEncodingContainer.swift	77.39% <0%> (+4.45%)	⬆️
...XMLCoder/Encoder/XMLUnkeyedEncodingContainer.swift	29.41% <0%> (+6%)	⬆️
...XMLCoder/Decoder/XMLUnkeyedDecodingContainer.swift	37.57% <0%> (+22.9%)	⬆️

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