Better message when new cabal-install available
A common mistake new users of cabal-install seem to make is to run `cabal update`, see the message "Note: there is a new version of cabal-install available.", accordingly install a new version of cabal-install by doing `cabal install cabal-install`, neglect to make sure that the newly installed cabal-install executable is in their path, and continue to run the cabal-install executable they had before. The next time they do `cabal update`, they see the same message and become confused. This commit changes the message in question to be more informative and helpful. In particular, if the currently running version of cabal-install is older than the newest version available from hackage, the user will not be told simply that "there is a new version of cabal-install available", but rather will be informed exactly what new versions are available, and what version they are currently running, along with a hint to change their $PATH variable if it turns out they're not running the version of cabal-install they thought they were.