Cygwin setup.exe update

[fd00]

Cygwin setup.exe (C:\cygwin64\setup-x86_64.exe) is too old to install cygwin packages. Please update to latest (2.918)

https://ci.appveyor.com/project/fd00/ruby-chkbuild/builds/43589988/job/617ky0mypiiaxkbk

Starting cygwin install, version 2.900
User has backup/restore rights
Current Directory: C:\cygwin64\var\cache\setup
Could not open service McShield for query, start and stop. McAfee may not be installed, or we don't have access.
root: C:\cygwin64 system
Selected local directory: C:\cygwin64\var\cache\setup
net: Preconfig
site: http://cygwin.mirror.constant.com/
mbox Parse Errors:  line 12: The current ini file requires at least version 2.903 of setup.
Please download a newer version from https://cygwin.com/setup-x86_64.exe
unattended_mode is set at mbox: returning default value
Ending cygwin install
:
:
Command exited with code 1

[FeodorFitsner]

You can easily download the latest setup.exe during the build with the following script: https://github.com/appveyor/build-images/blob/master/scripts/Windows/install_cygwin.ps1#L10-L13

[fd00]

Thank you.