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.
"This executable" seems like someone somewhere could conceivably be confused about what "this" meant.

How about, "You are not currently running the latest version of cabal-install"? That seems quite precise (which first person would also accomplish, though I agree it sounds a bit unusual :) ).