Temporarily depend on wks/mmtk-core

There will be a change in the mmtk-core repository that changes the
root-scanning API.  The hacks that implemented object-enqueuing tracing
in this binding will no longer work because the core no longer exposes
the ProcessEdgesWork type to the binding.

We use the following mmtk-core repository to develop the changes for
making Ruby working again:

https://github.com/wks/mmtk-core/tree/ruby-friendly-tracing

Once the changes are merged back to the official repo, we will let the
mmtk-ruby binding depend on the official repo again.

Author: wks

mmtk/Cargo.toml

@@ -26,7 +26,7 @@ env_logger = "0.9.0"
 memoffset = { version = "0.6.5", features = ["unstable_const"] }
 
 # Uncomment this to use mmtk-core from the official repository.
-mmtk = { git = "https://github.com/mmtk/mmtk-core.git", features = ["is_mmtk_object"]}
+mmtk = { git = "https://github.com/wks/mmtk-core.git", branch = "ruby-friendly-tracing", features = ["is_mmtk_object"]}
 
 # Uncomment this to use mmtk-core from a local repository.
 #mmtk = { path = "../../mmtk-core", features = ["is_mmtk_object"] }