rust: relax im-rc dependency to allow minor updates

This "15.0.*" requirement came from 0d99778af68a and is now replaced with plain
"15.0".

AFAICS, it really should allow (but not necessarily require) im-rc 15.1 .

Narrow requirement requirements with wildcard in the version is not used in
other places.

rust/hg-core/Cargo.toml

@@ -14,7 +14,7 b' bytes-cast = "0.2"'
 byteorder = "1.3.4"
 derive_more = "0.99"
 home = "0.5"
-im-rc = "15.0.*"
+im-rc = "15.0"
 itertools = "0.9"
 lazy_static = "1.4.0"
 libc = "0.2"