Rename flux to f-lux

See https://github.com/phinze/homebrew-cask/pull/146#discussion-diff-2872101
for details and the reason for the renaming. To summarise:
`f-lux` is as close as we can get to `f.lux`, and at the same
time we're freeing the spot for another app name `Flux`.

Signed-off-by: Felix Saparelli <me@passcod.name>

Casks/flux.rb → Casks/f-lux.rb

@@ -1,4 +1,4 @@
-class Flux < Cask
+class FLux < Cask
   url 'https://secure.herf.org/flux/Flux.zip'
   homepage 'http://stereopsis.com/flux/'
   version 'latest'