Add conda update --all

This works by taking all packages currently installed, and adding "package
>=X" to the specs, where X is the version of the package already installed. It
then passes this to the SAT solver. This prevents "update" from actually
downgrading. This also has a nice side-effect of making all currently
installed packages satisfiable.

Issues:

- No special handling of Python 2. If you use this on a Python 2 environment
  you will be upgraded to Python 3.
- Maybe the error message won't be so helpful in the unsatisfiable case (I
  don't know).
- If you have anaconda installed, it won't work, because anaconda has strict
  version requirements that prevent upgrades.

Fixes #147.