Manifest file identification isn't restrictive enough

[nkrabben]

manifest.rb matches manifest files with the following pattern /^manifest.*.txt/ https://github.com/tipr/bagit/blob/master/lib/bagit/manifest.rb#L19

If there is a file in the bag directory with a name like manifest-md5.txt.old, this is matched as a manifest file, although it shouldn't be according to the spec.

I think this can be fixed by using /^manifest.*.txt$/ as the match pattern instead.

The same change can be made to the tag-manifest filename test.