Warn about strange debian dependencies, but don't auto-fix (downcase) them

I tried to create a .deb for a custom application that depends on the strangely-named `libMagic++5` package.
As of fpm 1.2.0, the dependency name will be force-downcased (and a warning emitted), which results in a package that I cannot install (since there is no package named `libmagic++5` in the debian archive).

This modest PR keeps the warning but will not mangle the package name in the depend string anymore.