Test compatibility with new Co-Authors Plus release

[benlk]

https://github.com/Automattic/Co-Authors-Plus/releases/tag/3.4

[benlk]

• test locally
• check to see whether Travis automated tests pass for this
• write up a formal list of things to test

[benlk]

List of things for testing new Co-Authors Plus:

• post with WordPress author
• post with mixed authorship
  • order of WP_User/Co-Author set in post editor is preserved in byline output
  • names are joined by commas or 'and' as appropriate
• post with solely co-authors
  • order of WP_User/Co-Author set in post editor is preserved in byline output
  • names are joined by commas or 'and' as appropriate
• co-author author archive
  • co-author thumbnail/bio/social are displayed as with WordPress users
  • no posts not associated with the co-author are displayed
  • If first post on an author archive has multiple authors, the archive's heading displays both authors' bios. #1539 does not occur

[benlk]

Travis testing will come after the pull requests fixing #1720, #1721, #1722 are opened, and Travis ran against them.

[benlk]

#1720: fixed in #1726
#1721: patched in #1724, but needs a bigger-picture fix in #1725.
#1722: not actually a Largo problem
Travis: passing
CAP testing docs: still need to be written

[benlk]

PR #1729 adds the CAP testing docs, and a line about checking the plugin testing docs has been added to #1704