Fixes rbenv component path in the manifest files

Commit bc5a7c6be77b12e5181e3af37d749b836da9d759 changed the way rbenv is
installed ("Use the same Git/GitHub approach for Mac as we do for Linux."), but
was missing the changes in the manifest files. This commit fixes the
corresponding manifest files:

* changes mac-components/rbenv to common-components/rbenv in Manifest.mac
* changes linux-components/rbenv to common-components/rbenv in Manifest.linux