Refactors InstallCommand to use DefaultBuildSettings.

I've added DefaultBuildSettings class and moved `enablevariants` method call in InstallCommand to DefaultBuildSettings.
Since we have `DefaultBuildSettings` for now, we might want more build settings in the future, for example, UbuntuBuildSettings ... and so on.  so it would be great if we can have a separated namespace `PhpBrew\BuildSettings` for these classes.