Clean up build log messages

There's no need to keep repeating the path of the build script.
It just clutters the logs.
Allow a custom SWI-Prolog build

We sometimes have to run a version of SWI-Prolog with locally applied
patches.  To allow these kinds of situations, let users specify a custom
tarball URL in their dotcloud.yml file.
Log when dependency installation is finished

dotCloud sometimes pauses for many seconds after dependencies have
finished installing.  This new log messages communicates that the delay
is not caused by dependency installation.