Use the newer format of weighted-search-terms

    * Total monthly searches => Monthly searches
    * ignore incomplete rows
    * updated with the latest data
Be stricter when parsing numbers from the CSV

Integer("foo") will raise exceptions when it's not clear what the intended
number would be, whereas "foo".to_i will make a best effort,
eg "".to_i => 0
eg Integer("") => ArgumentError: invalid value for Integer()

This change should make it clearer when there is unintended data in the input.
Merge pull request #2 from alphagov/use_newer_weighted_format

Use the newer format of weighted-search-terms