Release stringr 1.3.1

[hadley]

• rhub::check_for_cran()
• devtools::check_win_devel()
• revdepcheck::revdep_check(num_workers = 4)
• Update NEWS and add breaking changes section
• Inform revdep maintainers
• Bump version (in DESCRIPTION and NEWS)
• devtools::check_win_devel()
• Submit to CRAN
• Approve email
• Update site
• Tag release
• Write blog post - skipping because patch release
• Tweet- skipping because patch release

(also update r-lib/usethis#338 if you update this list)