Remove $ from README and update install instructions #95

Job	Run time
test (ubuntu-latest, 2.7)	1m 18s
test (ubuntu-latest, 3.0)	30s
test (ubuntu-latest, 3.1)	29s
test (ubuntu-latest, head)	1m 33s
test (ubuntu-latest, jruby)	1m 17s
test (ubuntu-latest, jruby-head)	1m 11s
test (ubuntu-latest, truffleruby)	2m 19s
test (ubuntu-latest, truffleruby-head)	2m 39s
test (macos-latest, 2.7)	1m 0s
test (macos-latest, 3.0)	30s
test (macos-latest, 3.1)	36s
test (macos-latest, head)	1m 25s
test (macos-latest, jruby)	1m 15s
test (macos-latest, jruby-head)	1m 5s
test (macos-latest, truffleruby)	1m 45s
test (macos-latest, truffleruby-head)	1m 53s
	20m 45s

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