Added a mach command for cargo update

As proposed in #3736

Example usage:

``` sh
$ ./mach update-cargo -p rust-xml
.
    Updating git repository `https://github.com/netvl/rust-xml`
ports/cef
    Updating git repository `https://github.com/netvl/rust-xml`
ports/android/glut_app
    Updating git repository `https://github.com/netvl/rust-xml`
```

python/servo/devenv_commands.py

@@ -1,4 +1,5 @@
 from __future__ import print_function, unicode_literals
+from os import path
 
 import subprocess
 
@@ -8,7 +9,7 @@ from mach.decorators import (
     Command,
 )
 
-from servo.command_base import CommandBase
+from servo.command_base import CommandBase, cd
 
 
 @CommandProvider
@@ -24,6 +25,24 @@ class MachCommands(CommandBase):
         return subprocess.call(["cargo"] + params,
                                env=self.build_env())
 
+    @Command('update-cargo',
+             description='Update Cargo dependencies',
+             category='devenv',
+             allow_all_args=True)
+    @CommandArgument(
+        'params', default=None, nargs='...',
+        help='Command-line arguments to be passed through to cargo update')
+    def update_cargo(self, params):
+        cargo_paths = [path.join('.'),
+                       path.join('ports', 'cef'),
+                       path.join('ports', 'android', 'glut_app')]
+
+        for cargo_path in cargo_paths:
+            with cd(cargo_path):
+                print(cargo_path)
+                subprocess.call(["cargo", "update"] + params,
+                                env=self.build_env())
+
     @Command('rustc',
              description='Run the Rust compiler',
              category='devenv',