Fix: Linux Compile Error

[Nonchalant]

Fix

https://github.com/httpswift/swifter/releases/tag/1.4.0 has below error.

/SwiftBot/.build/checkouts/swifter.git--4815576737049089348/Sources/MimeTypes.swift:139:17: error: cannot convert value of type 'String' to type 'NSString' in coercion
        return (self as NSString).mimeType()
                ^~~~

I replace stable branch to fix it.
https://github.com/httpswift/swifter/tree/stable

Confirmation (Docker)

$ docker build .

Dockerfile

FROM swift:4.0.3

USER root
COPY ./Sources /SKServer/Sources
COPY ./Package.swift /SKServer/Package.swift

WORKDIR /SKServer
RUN swift build

[pvzig]

Great, thanks!