Bump Emacs version to 26.1.91

* README:
* configure.ac:
* msdos/sed2v2.inp:
* nt/README.W32: Bump Emacs version to 26.1.91.
1 job for emacs-26 in 27 minutes and 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #501
test

00:27:31

 
Name Stage Failure
failed
test Test
Makefile:285: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:255: recipe for target 'check' failed
make[1]: *** [check] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:951: recipe for target 'check' failed
make: *** [check] Error 2
ERROR: Job failed: exit code 1