Bubble legend: drop smallest category if zero

[grantmcdermott]

Fixes #497

devtools::load_all()
library("gapminder")

with(
  subset(gapminder, year == 2007),
  plt(
    x = gdpPercap, y = lifeExp,
    by = continent, facet = continent,
    pch = 21, bg = 0.4,
    cex = pop, clim = c(0.5, 3.5),
    main = "Gapminder!",
    theme = "clean"
  )
)

[grantmcdermott]

Need to update the tests (which are failing b/c of this precise issue). Will try to do tomorrow.