Update to 4.0.7

CHANGELOG:
4.0.7
-----
* Removed dependency on `constraints`. It was used in a pre-release version of
  4.0 but never made it into 4.0, but the dependency had remained around
  complicating builds for GHC 7.4.

4.0.6
-----
* `makeLenses` attempt to make the accessors it can under existential
  quantification.
* Added `(&~)`.
* _Experimental_ support for parallel builds on GHC 7.8 with
  `cabal install lens -fj`. Due to at last one known issue with GHC, it isn't
  recommended to use this option when rebuilding lens, as a race condition on
  at least one platform has been seen in the wild.
* Added `RoleAnnotations` for GHC 7.8.1. These rule out a few user-accessible
  bottoms that could be caused by creative abuse of the new `Coercible`
  machinery. However, there was no `unsafeCoerce` exposed.
* Removed some impossible cases that required unwritable instances from
  the example doctypes.
Update to 0.20.1

Release notes:
## Version 0.20.1 (March 13, 2014)
_New features_

  - Back to picking by default the first available screen, with a new
    configuration option, `pickBroadest`, for choosing the broadest
    (see [issue #158]).

[issue #158]: https://github.com/jaor/xmobar/issues/158