Check to ensure file contents are not fresher before applying fixes.

[SiriusStarr]

This might be marginal, so obviously feel free to disagree and close the issue, but it might be advisable to check (upon hitting y to apply a fix) that the file hasn't been modified since generating the fix. If one runs elm-review, leaves it open on the "apply fix" prompt, edits the file, and then applies the fix, the file edits will be clobbered.