Resolve issue with install via Swift Package Manager

[rcasula]

This fixes #47

[codecov]

Codecov Report

Merging #48 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #48   +/-   ##
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  Coverage     100%   100%           
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  Files           4      4           
  Lines          75     75           
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  Hits           75     75

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[yannickl]

@robcas3 thank you for your help. Your commit does not fix #47 but it only bumps the PromiseKit version.

[rcasula]

Well, thanks @yannickl.
But now I can install it from Xcode 11 beta 3, for me fixes the problem.
Why you said that does not fixes it?

The PromiseKit version bump was a side effect.
The real fix was to modify the PK Version in the changelog from 6.7 to 6.7.0.
For some inexplicable reason Xcode seems to read the changelog.

[yannickl]

@robcas3 it was my fault. It only misses the SPM version in the package but it was working. It was late and I was tired. :$