Un-break el-get when installing rust-mode

rust-mode no longer requires cm-mode, and furthermore, cm-mode has
been removed from the Rust repository.  That means that without this
patch updating one's rust-mode will break one's .emacs initialization.

Fix this by simply removing the dependency metadata.
Merge pull request #1318 from fdr/fix-with-rust-mode

Un-break el-get when installing rust-mode