Tweak Control.Lens import for better compatibility

Tweak Control.Lens import for better compatibility

One of the CI runs blowing up:

    /home/travis/build/wincent/docvim/lib/Text/Docvim/Optimize.hs:3:1: Warning:
        Module ‘Control.Lens’ does not export ‘Empty’
    <no location info>:
    Failing due to -Werror.

Source: https://travis-ci.org/wincent/docvim/jobs/138094062

I'm going to turn off `-Werror` in CI and keep it locally, but in the meantime I
think I can get compatibility with a wider range of Lens versions if I switch
the import to not mention `Empty`.

See also: https://github.com/wincent/docvim/issues/24