Removed Environment#bind

Environment#bind was simply indirection for a call to
`#browser_factory.bind`. Removing it simplifies the API and keeps the
form for bindings on the current browser factory congruent with that of
an explicit factory.

  env.browser_factory(:chrome).bind(...) # just for chrome
  env.browser_factory.bind(...) # for whichever factory we're using

Versus

  env.bind(:chrome, ...) # just for chrome
  env.browser_factory.bind(...) # for whichever we're using

Change-Id: I50ed539008ce11449c57a3f5a5ee9f02695a3713