I am currently following the document from swift.org to play around with the new Swift Package Manager.
I cloned the demo project from Github and run the following comma
You did not add the newly installed swift to your PATH. The instructions for doing that are here.
PATH
On OS X:
export PATH=/Library/Developer/Toolchains/swift-latest.xctoolchain/usr/bin:"${PATH}"
On Linux:
export PATH=/path/to/Swift/usr/bin:"${PATH}"
Then to test it works:
swift build --version