consider recipes with :checksum as "secure"

If the package recipe has a :checksum property no more work is needed:
either the checksum matches in which case we have a trusted version of
the package, or it fails to match (or doesn't have a :compute-checksum
method) so we get an error raised on el-get-post-install.

* el-get-methods.el (el-get-insecure-check): don't raise error if
  package recipe has :checksum property.