Bumps ffi from 1.9.6 to 1.11.1.

Sourced from ffi's changelog.

1.11.1 / 2019-05-20

Changed:

• Raise required ruby version to >=2.0. #699, #700
• Fix a possible linker error on ruby < 2.3 on Linux.

1.11.0 / 2019-05-17

This version was yanked on 2019-05-20 to fix an install issue on ruby-1.9.3. #700

Added:

• Add ability to disable or force use of system libffi. #669
  Use like gem inst ffi -- --enable-system-libffi .
• Add ability to call FFI callbacks from outside of FFI call frame. #584
• Add proper documentation to FFI::Generator and ::Task
• Add gemspec metadata. #696, #698

Changed:

• Fix stdcall on Win32. #649, #669
• Fix load paths for FFI::Generator::Task
• Fix FFI::Pointer#read_string(0) to return a binary String. #692
• Fix benchmark suite so that it runs on ruby-2.x
• Move FFI::Platform::CPU from C to Ruby. #663
• Move FFI::StructByReference to Ruby. #681
• Move FFI::DataConverter to Ruby (#661)
• Various cleanups and improvements of specs and benchmarks

Removed:

• Remove ruby-1.8 and 1.9 compatibility code. #683
• Remove unused spec files. #684

1.10.0 / 2019-01-06

Added:

• Add /opt/local/lib/ to ffi's fallback library search path. #638
• Add binary gem support for ruby-2.6 on Windows
• Add FreeBSD on AArch64 and ARM support. #644
• Add FFI::LastError.winapi_error on Windows native or Cygwin. #633

Changed:

• Update to rake-compiler-dock-0.7.0
• Use 64-bit inodes on FreeBSD >= 12. #644
• Switch time_t and suseconds_t types to long on FreeBSD. #627
• Make register_t long_long on 64-bit FreeBSD. #644
• Fix Pointer#write_array_of_type #637

... (truncated)

• 3d8c91c Update CHANGELOG for ffi-1.11.1
• 2a3d9d6 Bump VERSION to 1.11.1
• 19d5776 Add some missing items to CHANGELOG for ffi-1.11.0
• 08b9b78 Raise ruby version requirement to >=2.0
• e76c43b Fix build on ruby-2.0 on Linux
• 9b4a9e4 Add version 1.11.0 to CHANGELOG
• 36f1bb5 Bump VERSION to 1.11.0
• 29c07a9 Pass options from rake task to the FFI::Generator
• 7efdd2e Include the name of the template file instead of just rake
• bc99b7e Add proper documentation to FFI::Generator and ::Task
• Additional commits viewable in compare view

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