Don't duplicate 'DISTINCT' SQL keyword

`uniq`, when in the AR query context, adds the
`DISTINCT` SQL keyword to your queries. Explicitly
setting the `select` with a distinct and calling
`uniq` is redundant. Further, pluck is better when
only querying for a single column as it prevents
the full loading of AR model instances.

However, the `pluck` methos was not available in
this context. I was getting this error in the Chunk
Helper specs:

    Failure/Error: GoogleSafeBrowsing::ChunkHelper.build_chunk_list.
    NoMethodError:
      undefined method `pluck' for #<Class:0x007fb178a50380>
    # ./lib/google_safe_browsing/chunk_helper.rb:22:in `block in build_chunk_list'
    # ./lib/google_safe_browsing/chunk_helper.rb:11:in `each'
    # ./lib/google_safe_browsing/chunk_helper.rb:11:in `build_chunk_list'
    # ./spec/chunk_helper_spec.rb:15:in `block (4 levels) in <top (required)>'
FullHashes can delete themeseves when "subbed"

When a sub chunk is added for an already fetched
FullHash, that full hash should no longer be
matched, as it's removed from the database. Thus
we should delete them from our local cache of full
hashes.
Delete subbed FullHashes when receiving db updates
Remove dummy dev log file
Note that update must be run several times
Clean up some of the wrapping in README