Use new API for build script

[roya1v]

Changed the build script to use the new Task API as described in issue #392

[0xTim]

@jroya1 can you test the script first? It doesn't currently compile

scripts/build.swift:48:24: error: cannot assign value of type 'String' to type 'URL?'
    task.executableURL = "/usr/bin/env"
                       ^
scripts/build.swift:61:24: error: cannot assign value of type 'String' to type 'URL?'
    task.executableURL = "/usr/bin/env"
                       ^

[roya1v]

@0xTim Sorry, silly me being too confident :) Should be working now.

[0xTim]

Thanks!