Color scale buckets in hex map template don't always work properly

[brmayes]

When making the graphic minimum-wage-2018-increases-20171226, I noticed a problem I and other members of the team have had in the past: the color buckets weren't aligning properly when using the isNumeric flag.

@alykat has fixed this problem before by changing how she rounds in the spreadsheet. She fixed this by using floor and setting the numbers to round down instead of up.

That didn't work in my case. The fix that @ghing and I came up with was switching the scale from an ordinal scale to threshold and pushing the categories as floats.

Not high priority, but might be worth looking into as an update if we find that problem happening frequently.

[alykat]

Related: #203