Update CI configuration for the new vapor scripts

[vzsg]

The swift.vapor.sh domain disappeared, so tests on both Circle and Travis simply pass without doing anything.

The sudo-enabled Linux image on Travis is a mess, so from now on:

• Circle will test the package on Linux – with the new scripts;
• Travis will only test on macOS – as standalone as possible.

[codecov[bot]]

Codecov Report

Merging #49 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #49   +/-   ##
=======================================
  Coverage   80.25%   80.25%           
=======================================
  Files          10       10           
  Lines        1028     1028           
=======================================
  Hits          825      825           
  Misses        203      203

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