Ensure HashHelper converts raw hashes to gsb hashes correctly
Convert raw hashes to gsb hashes
Rename urls_to_hashes method

* The new method name (urls_to_gsb_hashes) is
more explicit about the fact that urls are converted
to GsbHashes in this method.

* Also, map method is used for aggregation instead of each.
Fix invalid SQL statement for collecting unique chunk numbers

* The select method on SubShavar already specifies 'distinct
chunk_number'. Therefore, calling uniq on the result set creates
an invalid SQL statement with 'DISTINCT' command included twice.

* For some reason, the tests pass in master branch without this commit.
I haven't figured out why it doesn't fail. I caught this because the tests
were failing in my branch due to duplicate 'DISTINCT' command in SQL
statement generated.
Use expect instead of should in HashHelper specs

expect provides more readable syntax for setting expectations.
https://github.com/rspec/rspec-expectations/pull/119
Ensure lookup_url_hashes returns nil if called with empty hashes
Allow looking url hashes up for badness

* Common logic between lookup and lookup_url_hashes
methods is extracted into private class methods.

* Variable renamed to increase readability of the overall code.
Don't load active_resource/railtie in dummy rails app

Rails 4.0 complains if the dummy app attempts to
load active_resource/railtie since it can not find that file.
However, Rails 3.x doesn't complain.
Document lookup_url_hashes method in readme
Bump gem version to 0.6.0.

Also, update Gemfile.lock with bundle update.