Precompiled binaries no longer support macOS 10.7

Since we compile with Qt 5.7, macOS 10.8 is the minimum requirement
for the binary packages.

Change-Id: Id9a08739e8f720399960735fd3b2072320f154fe
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

README.md

@@ -8,7 +8,7 @@ The standalone binary packages support the following platforms:
 
 Windows 7 or later
 (K)Ubuntu Linux 14.04 (64-bit) or later
-Mac OS X 10.7 or later
+macOS 10.8 or later
 
 Building the sources requires Qt 5.5.0 or later.