Update ruby-facter to 2.5.1.

2.5.0 (2017/07/05)
* (FACT-1024) Fix AWS EC2 Gzip userdata
* (FACT-1339) Prevent swap warnings on OpenBSD.
* (FACT-1395) Use Windows API for Windows version
* (FACT-1043) Fix virtual fact on Openstack Windows instances
* (FACT-1367) add "VirtuozzoLinux" to RedHat OS Family
* (FACT-1372) IPv6 are incorrectly parsed.
* (FACT-1478) Increase EC2 REST reachability timeout to 0.5s
* (FACT-1479) Replace Timeout.timeout with conventional read timeout
* (FACT-1488) Add git pack config on Windows
* (maint) Sign packages with new signing key
* (FACT-1505) Update URL for new release naming
* Disable proxy when fetching metadata.

2.5.1 (2017/08/17)
* (maint) Minor fixes to get validation working
* (packaging) Stop building all the things
* (FACT-1726) Return correct operatingsystemrelease value for Windows 2016
* (FACT-1729) Remove FFI attach_function_private
* Sort out FFI related methods.

(taca)