CRAN v0.5.6

CRAN 0.5.6 - Patch to address the following issues:

• dplyr joins with same variable names, see tidyverse/dplyr#2825
• Import dplyr 0.7 to avoid manual install of dbplyr
• sample_n performance #271
• Fix for #744 by avoiding override of now supported LIMIT statement, see: 72b2b97
• Workaround for tidyverse/dplyr#2848 and #744 see 939dc0a