Require MahApps.Metro 0.10.0.0 exactly

MahApps.Metro > 0.10.0.0 (ex, current latest 0.10.1.21) changed the assembly versions from 0.9.0 to 0.10.0, so references in Caliburn.Metro are no longer valid.
Merge pull request #1 from rexm/master

Current nuget package is breaking; fixed to require specific version