use equally spaced grid of values when generating colorscale

[cpsievert]

Closes #1308.

TESTING NOTES

On either linux or windows, please do:

# devtools::install_github("ropensci/plotly#1389")
library(plotly)
library(viridis)

f <- rnorm(100)
f[[5]] <- 1e2
plot_ly(x=f, y=f, color=f, colors=viridis(100))

You should see something like this (refer to #1308 for the 'incorrect' version):

[rich-iannone]

This looks good and I reproduce the plot you get exactly. I also tried other variations with the points spread out evenly, works well there as well. I do get some messages (helpful) in the console:

No trace type specified:
  Based on info supplied, a 'scatter' trace seems appropriate.
  Read more about this trace type -> https://plot.ly/r/reference/#scatter
No scatter mode specifed:
  Setting the mode to markers
  Read more about this attribute -> https://plot.ly/r/reference/#scatter-mode

which I'm guessing is entirely intentional for this minimal example.