Pin mojo pub version at 0.4.6.

v0.4.7 was released on Dec 9, which is incompatible with the mojo
version installed in our mojo profile.

Change-Id: I5124c2e618e5323b028dc136c888c61262a32539
diff --git a/pubspec.yaml b/pubspec.yaml
index d520612..1d36e3d 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@
 name: syncbase
 version: 0.0.23
 dependencies:
-  mojo: '>=0.4.3 <0.5.0'
+  mojo: 0.4.6
 dev_dependencies:
   dart_style: any
   test: any