port to time-1.5

ehem... you might want to consider using pvp ;-)
> Do know if there is a dual problem, if we set it to false, will it try
> to install time-1.4 even when time-1.5 is already installed?

well. my limited tests worked. all examples below have a local checkout 
with the flag defaulting to false.

the first test (within that checkout) it won't install time-1.4.

```shell
% cabal sandbox init
% cabal install time-1.5
% cabal install --dry-run
```

in the second test (in a project depending on `iso8601-time`) it will 
set the flag to true automatically.

```shell
% cabal sandbox add-source ../iso8601-time
% cabal install --dry-run
In order, the following would be installed:
iso8601-time-0.1.2 +new-time
```

the same happens with a local repository containing a new 
`iso8601-time`. (bumping the version number, adding it to a `local-repo` 
in cabal, etc.)


> Also, what do you think about the "depend on it anyway" solution?

i meant, not using a flag at all and depend on `old-locale` without 
using it.