plyr still used some places

[tdhock]

would be good to convert these uses of plyr to data.table and then remove plyr from Suggests.

grep  -nH --null plyr *
helper-plot-data.r:6:    plyr::ddply(d, "PANEL", function(panel_data) {
test-compiler-animation.R:4:library(plyr)
test-compiler-fortify.r:35:  expect_equivalent(fortify(fake_sp), plyr::arrange(fortify(fake_sp2), id, order))
test-compiler-save-separate-chunks.R:2:library(plyr)
test-renderer1-hjust-text-anchor.R:43:objective <- plyr::ldply(objective$iteration, function(i) {
test-renderer1-interactivity.R:181:library(plyr)