pass io.ReadCloser instances instead of files on harddrive, fixes #536

[m29h]

• before each list Downloader was creating temporary with files on disk that were subsequently read in by the list Parser
• this changes this "communication" to io.ReadCloser which is more ideomatic in Go
• fixes #536 as no temporary files clutter the disk any more

[m29h]

Fixed code linting problem... make check passes now in my environment

@adamdecaf can you rerun tests please

[adamdecaf]

Releasing this in https://github.com/moov-io/watchman/releases/tag/v0.28.1