Include directory entries to %files section for .rpm.

:rpm_auto_add_directories? attribute for fpm manages .rpm to have
both directory and file entries in %files. For the current behavior,
the generated .rpm leaves some empty folders on filesystem when it
got removed by "rpm -e" or "yum remove".
Merge pull request #67 from unakatsuo/enable-auto-add-dir

RPM should have directory entries in %files section as default.
Fix test description.
Port tests from minispec to rspec.
Use let instead of a method definition.
Pass config object to recipe constructor.
Add cli options for tmp-root, pkg-dir and cache-dir.

This allows to overwrite the default directories for the temp
build/install directories and the cache and package output dir.

Should help with #66.
Remove Ruby 2.1.1 from allowed failures in travis config.

[ci skip]
Add changelog for 0.20.0.
Bump version to 0.20.0.

Changes:

* Add --tmp-root command line option.
* Add --pkg-dir command line option.
* Add --cache-dir command line option.
* Fix %files section in rpms. (unakatsuo / #67)
Unbreak omnibus and chain packager.

Fixes #69
Add changelog for 0.20.1.
Bump version to 0.20.1.

Changes:

* Unbreak omnibus and chain packagers. (#69)