fix(coreos-base/oem-vagrant): Update minimum version to 1.6.0

Commits 24cdca8e and 9d2c0169 added options only in 1.6 and later,
breaking compatibility with older versions. Update the minimum version
to make the resulting error more obvious. :)

The coreos-vagrant project already required 1.6 (although not
explicitly, probably should fix that) so most uses probably won't notice.