devcam: warn when outdated

http://camlistore.org/issue/482

Change-Id: Ifafec9c9ff337aefa98e630242cb28f19495d4b5
New pkg/depcheck package to check Go version.

Based on kr's example (syntantically valid Go code, to make cmd/go happy, but
will fail to compile and include the failure text)

Change-Id: I11f9eeabf6bf2c77055752014c71183b7eb7b23a