Do not change $LOAD_PATH in psych.gemspec

[mame]

@deivid-rodriguez @tenderlove

The change for #433, which has been merged into ruby/ruby, produces dozens of warnings during make test-all in ruby master.

http://rubyci.s3.amazonaws.com/ubuntu2004/ruby-master/log/20200608T123004Z.log.html.gz

[15092/20234]
TestRDocOptions#test_init_with_encoding/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/ext/psych/lib/psych.rb:233: warning: already initialized constant Psych::LIBYAML_VERSION
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/.ext/common/psych.rb:233: warning: previous definition of LIBYAML_VERSION was here
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/ext/psych/lib/psych.rb:235: warning: already initialized constant Psych::NOT_GIVEN
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/.ext/common/psych.rb:235: warning: previous definition of NOT_GIVEN was here
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/ext/psych/lib/psych.rb:271: warning: method redefined; discarding old load
/home/chkbuild/chkbuild/tmp/build/20200608T123004Z/ruby/.ext/common/psych.rb:271: warning: previous definition of load was here
...

This is caused by double requiring ext/psych/lib/psych.rb and its copy file .ext/common/psych.rb.

A similar issue used to happen in fiddle. ruby/fiddle#34

I'm monitoring the number of warnings during test, so this is a bit noisy. Tentatively, I committed the same fix as fiddle directly into ruby master to make my monitoring easy. You can just import the fix if you like. If not, please fix it in your way. (I'm not particular about how to fix.)

Thanks!

[tenderlove]

ah ya, that makes sense. I'll pull the fix from fiddle in to psych.

[deivid-rodriguez]

I try to fix one redefinition warning and as a result, I create dozens of new instances of those 🤦. Thanks for fixing.

[tenderlove]

@deivid-rodriguez lol, no problem! Don't worry about it! 😆

[tenderlove]

I imported ruby/ruby@a3cc9b3 as 05d7e81, so this should be fixed