Require version 0.18 or later of CPAN::Changes.

Versions < 0.16 provide problems like this cpan-testers report:

http://www.cpantesters.org/cpan/report/9e4ab43c-44ca-11e3-99a8-9e2d15854db5

Versions < 0.18 of Test::CPAN::Changes do not provide their own versions
so you can't say 'use Test::CPAN::Changes 0.16;' because it does
export a version number.
Added bugtracker, changed indenting.
Upped version number, changed bug reporting info.
Improved .gitignore
Add tests for default mime-types

The default mime-types text/plain and application/octet-stream miss respective
tests. This is to test the different behavior of mimetype() and magic().
Fix Double-close on a filehandle (RT93221)

https://rt.cpan.org/Public/Bug/Display.html?id=93221

The problem is that _magic() closes the fd when it finds a result
for lower priority rules. mimetype() then tries to close it again.

Since we take a completely different route in mimetype() if
'ref file' is true, we can close the fd safely whenever the
low-priority rules return one.