Update deprecated dplyr functions

[hdrab127]

Hey there's quite a few changes here but they are all just replacing group_by_, mutate_, select_ etc. with their NSE equivalents (using !!, !!! where needed). And also replacing tbl_df with tibble::as_tibble as recommended by dplyr.

This removes all the warnings like

Warning messages:
1: group_by_() is deprecated as of dplyr 0.7.0.
Please use group_by() instead.
See vignette('programming') for more help

I've tested the examples on the web page and all seems to work, hopefully haven't missed anything.

Note: I didn't look at any of the files in dev/

When RStudio rebuilt the vignettes it failed the Mac tests, not sure what went wrong there sorry.

Thanks for the awesome repo!
Hayden

[jbkunst]

@hdrab127

Thanks so much for this PR!

Please send, if you want, a PR for add you to the package contributors in the DESCRIPTION file.