Updated neomutt to 20170428.

2017-04-28  Richard Russon  <rich@flatcap.org>
* Bug Fixes
  - Fix and simplify handling of GPGME in configure.ac (@gahr)
* Docs
  - Fix typo in README.neomutt (@l2dy)
* Upstream
  - Fix km_error_key() infinite loop and unget buffer pollution
  - Fix error message when opening a mailbox with no read permission

(wiz)