Make to load stringio lazily #306

Job	Run time
ruby-versions / Generate Ruby versions	2s
build (2.5, 0.1.7, /tmp/local)	51s
build (2.5, 0.1.7)	47s
build (2.5, 0.2.5, /tmp/local)	1m 0s
build (2.5, 0.2.5)	26s
build (2.6, 0.1.7, /tmp/local)	52s
build (2.6, 0.1.7)	28s
build (2.6, 0.2.5, /tmp/local)	1m 1s
build (2.6, 0.2.5)	45s
build (2.7, 0.1.7, /tmp/local)	50s
build (2.7, 0.1.7)	42s
build (2.7, 0.2.5, /tmp/local)	47s
build (2.7, 0.2.5)	28s
build (3.0, 0.1.7, /tmp/local)	37s
build (3.0, 0.1.7)	26s
build (3.0, 0.2.5, /tmp/local)	24s
build (3.0, 0.2.5)	17s
build (3.1, 0.1.7, /tmp/local)	36s
build (3.1, 0.1.7)	19s
build (3.1, 0.2.5, /tmp/local)	30s
build (3.1, 0.2.5)	18s
build (3.2, 0.1.7, /tmp/local)	25s
build (3.2, 0.1.7)	29s
build (3.2, 0.2.5, /tmp/local)	36s
build (3.2, 0.2.5)	30s
build (3.3, 0.1.7, /tmp/local)	35s
build (3.3, 0.1.7)	17s
build (3.3, 0.2.5, /tmp/local)	32s
build (3.3, 0.2.5)	21s
build (head, 0.1.7, /tmp/local)	21s
build (head, 0.1.7)	17s
build (head, 0.2.5, /tmp/local)	25s
build (head, 0.2.5)	19s
	17m 33s

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