install: Remove references to "Zulip Voyager".

"Zulip Voyager" was a name invented during the Hack Week to open
source Zulip for what a single-system Zulip server might be called, as
a Star Trek pun on the code it was based on, "Zulip Enterprise".

At the time, we just needed a name quickly, but it was never a good
name, just a placeholder.  This removes that placeholder name from
much of the codebase.  A bit more work will be required to transition
the `zulip::voyager` Puppet class, as that has some migration work
involved.

zproject/dev_urls.py

@@ -76,8 +76,7 @@ i18n_urls = [
     url(r'^confirmation_key/$', zerver.views.development.registration.confirmation_key),
 ]
 
-# These are used for voyager development. On a real voyager instance,
-# these files would be served by nginx.
+# On a production instance, these files would be served by nginx.
 if settings.LOCAL_UPLOADS_DIR is not None:
     urls += [
         url(r'^user_avatars/(?P<path>.*)$', serve,