Specify swift version

[vaind]

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Getting the following version during build. I can get around that by specifying the version in the consuming pod, but would be nice if the reason was fixed in the dependency itself:

    - `STRegex` does not specify a Swift version and none of the targets (`Runner`) integrating it have the `SWIFT_VERSION` attribute set. Please contact the author or set the `SWIFT_VERSION`
    attribute in at least one of the targets that integrate this pod.