bower2bazel: don't specify versions for non-seed packages.

The 'seed' packages are the ones whose versions are set by us in
WORKSPACE. We should not set the versions for the rest of the packages
in the bower input JSON, so bower can suggest the right versions to
use.

Change-Id: I9b75f16655d049e2064726862980a339c91dd534

tools/bzl/js.bzl

diff --git a/tools/bzl/js.bzl b/tools/bzl/js.bzl
index 1aa9d5b..af51af8 100644
--- a/tools/bzl/js.bzl
+++ b/tools/bzl/js.bzl
@@ -268,6 +268,15 @@
         "version_json": "%{name}-versions.json",
     },
 )
+"""Groups a set of bower components together in a zip file.
+
+Outputs:
+  NAME-versions.json:
+    a JSON file containing a PKG-NAME => PKG-NAME#VERSION mapping for the
+    transitive dependencies.
+  NAME.zip:
+    a zip file containing the transitive dependencies for this bundle.
+"""
 
 def _vulcanize_impl(ctx):
   # intermediate artifact.