Add "required IPM version" to SystemRequirements

[isc-tleavitt]

It should be possible to specify a semantic version expression in SystemRequirements that must be met by the currently-installed IPM version for the installation to work. This needs to be checked early in the process (before trying to install a package or dependency) so a different version can be found and used instead if the latest version won't work with the current IPM version.

This would have been convenient to have already, but we don't need it for 0.9.0 or 1.0 release.

Sample use case: zpm 0.9.0 should only be installable on zpm 0.7.1+, because on earlier versions it will fail to install.