Lock down apt cookbook to `< 2.5.0`

v2.5.0 replaced the `execute[apt-get update]` resource with
`execute[apt-get-update]` resource, which didn't earlier trigger on
every call. To avoid complexity, we now pin the version down. And soon
release a new major version which depends on apt cookbook 2.5 or later,
and thus drops support for Chef 10.

Ref: #15
Use the new supermarket link