CI: add bigdecimal to Gemfile #282

Job	Run time
coverage (2.7, ubuntu-20.04)	33s
profile (2.7, native, ubuntu-20.04)	1m 56s
profile (2.7, pure, ubuntu-20.04)	2m 0s
test (2.2, ubuntu-20.04, Gemfile)	32s
test (2.3, ubuntu-20.04, Gemfile)	31s
test (2.4, ubuntu-20.04, Gemfile)	30s
test (2.5, ubuntu-20.04, Gemfile)	30s
test (2.6, ubuntu-20.04, Gemfile)	31s
test (2.7, ubuntu-20.04, Gemfile)	31s
test (3.0, ubuntu-20.04, Gemfile)	22s
test (3.1, ubuntu-20.04, Gemfile)	22s
test (3.2, ubuntu-20.04, Gemfile)	24s
test (3.3, ubuntu-20.04, Gemfile)	24s
test (3.4.0-preview1, ubuntu-20.04, Gemfile)	18s
test (jruby-9.1, ubuntu-20.04, Gemfile)	37s
test (jruby-9.2, ubuntu-20.04, Gemfile)	51s
test (jruby-9.3, ubuntu-20.04, Gemfile)	35s
test (jruby-9.4, ubuntu-20.04, Gemfile)	38s
test (truffleruby-22.1, ubuntu-20.04, Gemfile)	2m 27s
test (truffleruby-22.2, ubuntu-20.04, Gemfile)	2m 14s
test (truffleruby-22.3, ubuntu-20.04, Gemfile)	2m 28s
test (ubuntu-20.04, 2.7, gemfiles/public_suffix_2.rb)	33s
test (ubuntu-20.04, 2.7, gemfiles/public_suffix_3.rb)	35s
test (ubuntu-20.04, 2.7, gemfiles/public_suffix_4.rb)	31s
test (ubuntu-22.04, 3.1)	32s
test (macos-11, 3.1)	1m 54s
test (macos-12, 3.1)	40s
test (windows-2019, 3.1)	2m 2s
test (windows-2022, 3.1)	1m 49s
test (windows-2022, jruby-9.3)	2m 12s
test (ubuntu-20.04, head, Gemfile, true)	18s
test (ubuntu-20.04, jruby-head, Gemfile, true)	39s
test (ubuntu-20.04, truffleruby-head, Gemfile, true)	1m 14s
	32m 13s

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