!fixup restore OCaml < 4.13 support #684

Job	Run time
Analyse	9s
Cygwin	7s
Hygiene	16s
Build-macOS (4.14.1)	2m 18s
Build-Linux (4.08.1)	4m 53s
Test-macOS (4.14.1)	23m 32s
Build-Linux (4.09.1)	5m 23s
Build-Linux (4.10.2)	4m 55s
Build-Linux (4.11.2)	4m 57s
Build-Linux (4.12.1)	4m 56s
Build-Linux (4.13.1)	5m 25s
Build-Linux (5.0.0)	5m 39s
Build-Linux (5.1.1)	5m 29s
Build-Linux (4.14.1)	5m 42s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	26m 49s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	35m 16s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	36m 13s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	40m 46s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	42m 30s
Upgrade-macOS (4.14.1)	1m 47s
Solvers-macOS (z3, 4.14.1)	1m 53s
Solvers-macOS (0install, 4.14.1)	1m 12s
Cold-Linux	4m 43s
Test-Linux (4.14.1)	16m 3s
Upgrade-Linux (4.14.1)	1m 10s
Solvers-Linux (z3, 4.14.1)	15m 52s
Solvers-Linux (0install, 4.14.1)	5m 48s
	5h 3m 43s

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