Add SWIG/Ruby compatibility check & warning

[ferdnyc]

Ruby 2.7.0 contains some SWIG incompatibilities that weren't addressed until the (unreleased) SWIG 4.0.3, so if we detect the new Ruby version, emit a warning when generating the build. (The warning can be silenced by the user, as indicated.)

-- Found Ruby: /usr/bin/ruby (found version "2.6.5") 
CMake Warning at src/bindings/ruby/CMakeLists.txt:51 (message):
  

  Ruby 2.7.0+ detected, building the libopenshot Ruby API bindings requires a
  pre-release version of SWIG 4.0.3 with this commit:
  https://github.com/swig/swig/commit/5542cc228ad10bdc5c91107afb77c808c43bf2a4


-- 
To disable this warning, add -DSILENCE_RUBY_VERSION_WARNING:BOOL=1 to the cmake
 command line, or enable the option in the CMake GUI.

(I had temporarily set the version comparison down to 2.6.0 to force the warning for this test, since I'm not running Ruby 2.7.0 yet.)

Fixes #479

[codecov-io]

Codecov Report

Merging #480 into develop will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #480   +/-   ##
========================================
  Coverage    48.13%   48.13%           
========================================
  Files          128      128           
  Lines         9933     9933           
========================================
  Hits          4781     4781           
  Misses        5152     5152           

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[ferdnyc]

Looks like this may have become a moot point, largely. I see that on both Fedora 32 and Ubuntu 20.04, SWIG 4.0.1 was subsequently patched to fix Ruby 2.7.0 support.