CI: add bigdecimal to Gemfile #283

Job	Run time
profile (2.7, native, ubuntu-20.04)	1m 54s
coverage (2.7, ubuntu-20.04)	36s
profile (2.7, pure, ubuntu-20.04)	2m 13s
test (2.2, ubuntu-20.04, Gemfile)	27s
test (2.3, ubuntu-20.04, Gemfile)	25s
test (2.4, ubuntu-20.04, Gemfile)	30s
test (2.5, ubuntu-20.04, Gemfile)	41s
test (2.6, ubuntu-20.04, Gemfile)	56s
test (2.7, ubuntu-20.04, Gemfile)	47s
test (3.0, ubuntu-20.04, Gemfile)	24s
test (3.1, ubuntu-20.04, Gemfile)	21s
test (3.2, ubuntu-20.04, Gemfile)	20s
test (3.3, ubuntu-20.04, Gemfile)	20s
test (jruby-9.1, ubuntu-20.04, Gemfile)	41s
test (jruby-9.2, ubuntu-20.04, Gemfile)	51s
test (jruby-9.3, ubuntu-20.04, Gemfile)	39s
test (jruby-9.4, ubuntu-20.04, Gemfile)	43s
test (truffleruby-22.1, ubuntu-20.04, Gemfile)	2m 16s
test (truffleruby-22.2, ubuntu-20.04, Gemfile)	2m 28s
test (truffleruby-22.3, ubuntu-20.04, Gemfile)	2m 20s
test (ubuntu-20.04, 2.7, gemfiles/public_suffix_2.rb)	37s
test (ubuntu-20.04, 2.7, gemfiles/public_suffix_3.rb)	45s
test (ubuntu-20.04, 2.7, gemfiles/public_suffix_4.rb)	29s
test (ubuntu-22.04, 3.1)	23s
test (macos-11, 3.1)	2m 2s
test (macos-12, 3.1)	27s
test (windows-2019, 3.1)	1m 32s
test (windows-2022, 3.1)	1m 51s
test (windows-2022, jruby-9.3)	1m 27s
test (ubuntu-20.04, head, Gemfile, true)	13s
test (ubuntu-20.04, jruby-head, Gemfile, true)	39s
test (ubuntu-20.04, truffleruby-head, Gemfile, true)	1m 0s
	31m 17s

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