Issue Description:

swift-build links against libcurl.so.4 provided by libcurl3, however, the Swift documentation neglects to mention this dependency.

Long term, it would be best to provide .DEB packages instead of raw tarballs, such as with fpm, to help automate dependency management on behalf of Ubuntu Swift users. That way, it's no longer a matter of keeping documentation up to date, but rather communicating this information via the package system, where tests, such as with Docker or Vagrant, can automatically verify accuracy.