Drop code for Ruby 1.6 #93

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (2.5, ubuntu-latest)	29s
test (2.5, windows-latest)	3m 28s
test (2.6, ubuntu-latest)	29s
test (2.6, macos-latest)	41s
test (2.6, windows-latest)	1m 56s
test (2.7, ubuntu-latest)	27s
test (2.7, macos-latest)	33s
test (2.7, windows-latest)	2m 28s
test (3.0, ubuntu-latest)	14s
test (3.0, macos-latest)	18s
test (3.0, windows-latest)	1m 24s
test (3.1, ubuntu-latest)	14s
test (3.1, macos-latest)	18s
test (3.1, windows-latest)	1m 28s
test (3.2, ubuntu-latest)	20s
test (3.2, macos-latest)	18s
test (3.2, windows-latest)	1m 21s
test (3.3, ubuntu-latest)	15s
test (3.3, macos-latest)	18s
test (3.3, windows-latest)	1m 59s
test (head, ubuntu-latest)	16s
test (head, macos-latest)	17s
test (head, windows-latest)	1m 25s
test (jruby, ubuntu-latest)	4m 22s
test (jruby-head, ubuntu-latest)	4m 21s
test (truffleruby, ubuntu-latest)	1m 35s
test (truffleruby-head, ubuntu-latest)	1m 56s
	33m 13s

Provide feedback