rust-dependencies: pin the dependencies of hg-core to prevent breakage

A new version of the `rand` crate broke two discovery tests. We should
upgrade our dependencies periodically manually.

Differential Revision: https://phab.mercurial-scm.org/D7115

rust/hg-core/Cargo.toml

@@ -12,6 +12,6 b' name = "hg"'
 byteorder = "1.3.1"
 lazy_static = "1.3.0"
 memchr = "2.2.0"
-rand = "> 0.6.4"
-rand_pcg = "> 0.1.0"
-regex = "^1.1"
+rand = "0.6.5"
+rand_pcg = "0.1.1"
+regex = "1.1.0"