Remove Gemfile, add script for Rubocop #1769

Job	Run time
RuboCop	31s
specs (ubuntu, 3.0.6)	1m 39s
specs (ubuntu, 3.1.4)	1m 34s
specs (ubuntu, 3.2.2)	1m 35s
specs (ubuntu, 3.3.0)	1m 29s
specs (macos, 3.0.6)	1m 21s
specs (macos, 3.1.4)	1m 23s
specs (macos, 3.2.2)	1m 7s
specs (macos, 3.3.0)	1m 20s
specs (windows, 3.0.6)	5m 23s
specs (windows, 3.1.4)	5m 41s
specs (windows, 3.2.2)	5m 53s
specs (windows, 3.3.0)	5m 45s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 32s
	36m 13s

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