1.5.15

This is probably the last release of the series of v1.5.x.
All changes are minor ones.

What's Changed

• Fix an unexpected error in qlot outdated due to missing a symbol import. by @fukamachi in #296
• Fix qlot outdated to show updates of the default Quicklisp dist. by @fukamachi in #298
• Allow --debug option for 'remove', 'check' and 'outdated'. by @fukamachi in #299

Full Changelog: 1.5.14...1.5.15