dplyr 0.4.2

This is a minor release containing fixes for a number of crashes and issues identified by R CMD CHECK. There is one new "feature": dplyr no longer complains about unrecognised attributes, and instead just copies them over to the output.

• lag() and lead() for grouped data were confused about indices and therefore
  produced wrong results (#925, #937). lag() once again overrides lag()
  instead of just the default method lag.default(). This is necesary due to
  changes in R CMD check. To use the lag function provided by another package,
  use pkg::lag.
• Fixed a number of memory issues identified by valgrind.
• Improved performance when working with large number of columns (#879).
• Lists-cols that contain data frames now print a slightly nicer summary
  (#1147)
• Set operations give more useful error message on incompatible data frames
  (#903).
• all.equal() gives the correct result when ignore_row_order is TRUE
  (#1065) and all.equal() correctly handles character missing values (#1095).
• bind_cols() always produces a tbl_df (#779).
• bind_rows() gains a test for a form of data frame corruption (#1074).
• bind_rows() and summarise() now handles complex columns (#933).
• Workaround for using the constructor of DataFrame on an unprotected object
  (#998)