Upgrade rails to 3.2.21
Be clearer on backslash usage.

See #4282
Fix #4414 & #4415 - exitfunc and proper null-terminated string

This patch fixes the following for messagebox.rb

Issue 1 (#4415)
When exitfunc is none, the payload will not be able to generate
due to an "invalid opcode" error.

Issue 2: (#4414)
After "user32.dll" is pushed onto the stack for the LoadLibrary
call, the payload does not actually ensure bl is a null byte, it
just assumes it is and uses it to modify the stack to get a
null-terminated string.

Fix #4414
Fix #4415
Define the Rails version constraint in a library constant
Fix jcxz to jecxz for x86_64 in metasm

This fixes "invalid opcode near 'jecxz'" for x64 metasm encoding.
Add new/updated r7 employees to mailmap
Add and sort other contributors in mailmap
Make the version constraint a range
Land #4434, fix for metasm invalid opcode
Lands #4429 by fixing windows/messagebox with EXITFUNC=none
Lands #4426 by cleaning up the module description
pull latest mdm
Land #4416, define rails version dep in one place

* Bump rails to 3.2.21
Fix #4430 - Add support for -o to save the payload to disk

Fix #4430
Land #4433 - Update mailmap
No point to save if there's no payload
Well, should be -1
Remove references to Redmine in code

See #4400. This should be all of them, except for, of course, the module
that targets Redmine itself.

Note that this also updates the README.md with more current information
as well.
Missed one in CONTRIBUTING.md
Lands #4436, latest version of MDM

Fixes an issue with protocol inclusion when adding an NX console