Bump upper version bound for lens to 4.5

Lens 4.4 drops the dependencies for aeson, attoparsec and scientific.

Release Notes: http://www.reddit.com/r/haskell/comments/2e9918/lens_44_release_notes/

I tested that vty compiles with lens-4.4 which it did but `cabal test` failed with an error:

    Could not find module ‘Distribution.Simple.Test.LibV09’
    Perhaps you meant Distribution.Simple.Test (from Cabal-1.18.1.3)
    Use -v to see a list of the files searched for.

Which is unrelated to the lens version AFAIK.

Best,
Markus