Do not prompt for SourceTree Updates when OS is incompatible with ST's newest version

Problem Definition

Users using an older version of Mac and SourceTree get alerts to update to the newest version of SourceTree. However, the newest version may not be supported in the old OS they have.

If the user then upgrades SourceTree, this upgraded version no longer works.

Suggested Solution

Check the user's OS and do not prompt to update if their OS is not supported for the latest version of SourceTree.
Additionally, if the user tries to upgrade SourceTree manually, give a warning for the fact that their OS is not compatible with the newest ST version.