Force `Kernel#open` to always return a `Tempfile`

This works around the fact that the OpenURI-provided `Kernel#open`
returns a `StringIO` OR a `Tempfile` instances depending on the size of
the data being downloaded. In the case of Omnibus we always want a
`Tempfile`.

Fixes #381
Merge pull request #399 from opscode/schisamo/open-uri-tempfile

Force `Kernel#open` to always return a `Tempfile`