Upgrade to cargo 0.16.0-nightly (built 2016-12-06)

Instead of once per day (or night) with a date in the URL,
Cargo binaries are now built and published for every merge to master
(with the merge commit hash in the URL).

python/servo/bootstrap_commands.py

@@ -295,7 +295,7 @@ class MachCommands(CommandBase):
         os.makedirs(cargo_dir)
 
         tgz_file = "cargo-nightly-%s.tar.gz" % host_triple()
-        nightly_url = "https://static-rust-lang-org.s3.amazonaws.com/cargo-dist/%s/%s" % \
+        nightly_url = "https://s3.amazonaws.com/rust-lang-ci/cargo-builds/%s/%s" % \
             (self.cargo_build_id(), tgz_file)
 
         download_file("Cargo nightly", nightly_url, tgz_file)

python/servo/command_base.py

@@ -336,7 +336,7 @@ class CommandBase(object):
 
     def cargo_build_id(self):
         if self._cargo_build_id is None:
-            filename = path.join(self.context.topdir, "cargo-nightly-build")
+            filename = path.join(self.context.topdir, "cargo-commit-hash")
             with open(filename) as f:
                 self._cargo_build_id = f.read().strip()
         return self._cargo_build_id