Prevent the confusion regarding Cabal version mismatch

[benarmston]

cabal-constraints must be built against the same version of Cabal as is being used to build the project it is being run against. However, it accepts both Cabal 1.16.* and 1.18.*. If it is built prior to the user installing Cabal 1.18 very odd error messages will be presented to the user. See issue #1 for an example of this.

Perhaps the solution is to require Cabal 1.18. This may not be such a bad idea as cabal-constraints is most useful when used with cabal sandboxes, and support for cabal.config both of which require cabal-install 1.18 and Cabal 1.18.