Fix CI failure caused by transitive dev dependency implicit upgrade

The newer version doesn't build on our MSRV, so we're just locking it to
the older version now.  Since this is just a dev dependency for
testing/benchmarking, this has no implications (e.g. bug fix, security)
for users of Ropey, so an older version should be fine.

Author: cessen

Cargo.toml

@@ -34,6 +34,11 @@ unicode-segmentation = "1.3"
 fnv = "1"
 fxhash = "0.2"
 
+# This is a transitive dependency of criterion--we don't use it directly. We
+# lock it to this exact version because newer versions don't work on our MSRV,
+# and thus CI fails if it's on a newer version.
+unicode-width = "=0.1.13"
+
 #-----------------------------------------
 
 [[bench]]