Remove $SRC_DIR so cached builds have files from latest commit

[sk33lz]

[Auspicus]

@sk33lz Submitted https://github.com/ProboCI/probo/pull/159.

Did this across all the source code downloader tasks. We discussed moving it to a higher level abstraction like "SourceCodeDownloader".

I figured because there's already duplication (these lines appear across all downloaders: https://github.com/ProboCI/probo/blob/master/lib/plugins/TaskRunner/BitbucketDownloader.js#L91-L92) that the work to move the downloader setup to an abstraction should probably be left for another PR.