Add Cabal and cabal-install version numbers when writing a new config file.

These values can be useful for debugging cabal issues.
Merge pull request #2152 from erikd/config

Add Cabal and cabal-install version numbers when writing a new config fi...
cabal-install/IndexUtils : Expose function getIndexFileAge.

* The function has type 'Repo -> IO Double'.
* It returns the age in days as a Double.
* Places that expected the age as an Int have been updated.
* Builds with ghc 7.4.2 and 7.8.3.
cabal-install/IndexUtils : Fix display of age now that its days as a Double.