Don't create symlink Xcode.app to installed version if Xcode does not already exist

[loyaltyarm]

If you are bootstrapping machines for development, they do not have Xcode.app installed out of the box. Using /usr/local/bin/xcversion install 8 installs Xcode-8.app and a symlink Xcode.app that points to the Xcode-8.app application bundle. This request is to ask that the symlink not take place, and Xcode.app remains as normal if there are no other versions present.

drwxr-xr-x 4 root wheel 136 Oct 12 00:38 Xcode-8.app
lrwxr-xr-x 1 root admin 25 Oct 11 00:00 Xcode.app -> /Applications/Xcode-8.app