[Security] Bump ffi from 1.9.23 to 1.9.25

[greysteil]

Bumps ffi from 1.9.23 to 1.9.25. This update includes security fixes.

Vulnerabilities fixed

Sourced from The GitHub Vulnerability Alert Database.

CVE-2018-1000201
See https://nvd.nist.gov/vuln/detail/CVE-2018-1000201.

Affected versions: < 1.9.24

Changelog

Sourced from ffi's changelog.

1.9.25 / 2018-06-03

Changed:

• Revert closures via libffi.
  This re-adds ClosurePool and fixes compat with SELinux enabled systems. #621

1.9.24 / 2018-06-02

Security Note:

This update addresses vulnerability CVE-2018-1000201: DLL loading issue which can be hijacked on Windows OS, when a Symbol is used as DLL name instead of a String. Found by Matthew Bush.

Added:

• Added a CHANGELOG file
• Add mips64(eb) support, and mips r6 support. (#601)

Changed:

• Update libffi to latest changes on master.
• Don't search in hardcoded /usr paths on Windows.
• Don't treat Symbol args different to Strings in ffi_lib.
• Make sure size_t is defined in Thread.c. Fixes #609

Commits

• aa1b844 Prepare for release 1.9.25
• f1385ae Revert "README: Remove now unnecessary PaX workaround [ci skip]"
• 94441aa Revert "Do closures via libffi"
• 4e1051a Run rspec with dots output only
• e70b13d Fix integer parameter range specs
• 55ae232 Fix several specs where raise_error was called without class
• 8821d4f Specify error class for several raise_error calls
• bf48d44 Fix missing C declarations causing compiler warnings
• f569788 Replace symlinks for mips r6 with plain files
• fedbae0 Update CHANGELOG
• Additional commits viewable in compare view

Looks like TimeOverflow isn't vulnerable in production here (the gem is a subdependency of selenium-webdriver), but still best practice and will silence any GitHub alerts you're getting.