Bump ffi from 1.9.18 to 1.15.4 in /docs

[dependabot[bot]]

Bumps ffi from 1.9.18 to 1.15.4.

Changelog

Sourced from ffi's changelog.

1.15.4 / 2021-09-01

Fixed:

• Fix build for uClibc. #913
• Correct module lookup when including ffi-module gem. #912

Changed:

• Use ruby code of the ffi gem in JRuby-9.2.20+. #915

1.15.3 / 2021-06-16

Fixed:

• Fix temporary packaging issue with libffi. #904

1.15.2 / 2021-06-16

Added:

• Add support for Windows MINGW-UCRT build. #903
• Add /opt/homebrew/lib/ to fallback search paths to improve homebrew support. #880 #882

Changed:

• Regenerate types.conf for FreeBSD12 aarch64. #902

1.15.1 / 2021-05-22

Fixed:

• Append -pthread to linker options. #893
• Use arm or aarch64 to identify Apple ARM CPU arch. #899
• Allow overriding gcc with the CC env var in const_generator.rb and struct_generator.rb. #897

1.15.0 / 2021-03-05

Fixed:

• Fix MSVC build
• Fix async callbacks in conjunction with fork(). #884

Added:

• Allow to pass callbacks in varargs. #885
• Name the threads for FFI callback dispatcher and async thread calls for easier debugging. #883 The name can be retrieved by Thread.name and is shown by Thread.list.inspect etc. Even gdb shows the thread name on supported operating systems.

... (truncated)

Commits

• 7429c5a Bump VERSION to 1.15.4 and add CHANGELOG entry
• cd0db90 Merge pull request #913 from xtkoba/uclibc
• 05eac54 Fix build for uClibc
• 7c70b82 Correct module lookup when including ffi-module
• 4d8fbd9 Allow prerelease FFI::VERSION
• 111d151 Remove unused rubygems-tasks development dependency
• 50098f2 Bump VERSION to 1.15.4.pre1
• 0dbbe5e Lower the JRuby version guard to 9.2.20
• 1880a71 CI: Install libffi which matches running arch (#905)
• 931efbe Bump VERSION to 1.15.3
• Additional commits viewable in compare view

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