Ensure strings with only underscores are not processed as Integer #314

Job	Run time
ruby-versions / Generate Ruby versions	2s
build (2.5, 0.1.7, /tmp/local)	45s
build (2.5, 0.1.7)	27s
build (2.5, 0.2.5, /tmp/local)	48s
build (2.5, 0.2.5)	26s
build (2.6, 0.1.7, /tmp/local)	48s
build (2.6, 0.1.7)	28s
build (2.6, 0.2.5, /tmp/local)	49s
build (2.6, 0.2.5)	30s
build (2.7, 0.1.7, /tmp/local)	48s
build (2.7, 0.1.7)	30s
build (2.7, 0.2.5, /tmp/local)	47s
build (2.7, 0.2.5)	29s
build (3.0, 0.1.7, /tmp/local)	23s
build (3.0, 0.1.7)	16s
build (3.0, 0.2.5, /tmp/local)	25s
build (3.0, 0.2.5)	18s
build (3.1, 0.1.7, /tmp/local)	20s
build (3.1, 0.1.7)	18s
build (3.1, 0.2.5, /tmp/local)	21s
build (3.1, 0.2.5)	18s
build (3.2, 0.1.7, /tmp/local)	24s
build (3.2, 0.1.7)	22s
build (3.2, 0.2.5, /tmp/local)	25s
build (3.2, 0.2.5)	17s
build (3.3, 0.1.7, /tmp/local)	22s
build (3.3, 0.1.7)	17s
build (3.3, 0.2.5, /tmp/local)	24s
build (3.3, 0.2.5)	22s
build (head, 0.1.7, /tmp/local)	21s
build (head, 0.1.7)	18s
build (head, 0.2.5, /tmp/local)	19s
build (head, 0.2.5)	15s
	14m 22s

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