Specify minor MacOS version before upgrade.

Bug: skia:7203
NoTry: true
Change-Id: I24a098bff3897e36cf2a6125980e842b37261a2a
Reviewed-on: https://skia-review.googlesource.com/67320
Reviewed-by: Stephan Altmueller <stephana@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
2 files changed