The 'requirements' key allows to specify the Debian packages which must
be installed and their required version. It must be an array of the
package name as the key and its version specifier - for its format, see
yunohost.utils.packages.Specifier - as value. For example:
"requirements": {
"yunohost": ">= 2.4, << 2.5"
}
It introduces a new `yunohost.utils` package which will contain common
utils used by YunoHost actions. It comes with a work in progress `packages`
module with simple methods to retrieve installed version of packages for now.
