Pre-release packages aren't handled correctly

[flagbug]

Creating a pre-release package, for example 1.0.0-beta1 and installing it, creates a folder called app-1.0.0 in the installation directory.

Also, incrementing the beta package to 1.0.0-beta2 doesn't work, it won't create a diff package and also can't be installed as it Squirrel tries to overwrite the existing app-1.0.0 directory.