Update support minimum version for OS X.

Author: QianGuoqiang

README.md

@@ -1,5 +1,5 @@
 # MMMarkdown
-MMMarkdown is an Objective-C framework for converting [Markdown][] to HTML. It is compatible with OS X 10.6+, iOS 8.0+, tvOS, and watchOS.
+MMMarkdown is an Objective-C framework for converting [Markdown][] to HTML. It is compatible with OS X 10.7+, iOS 8.0+, tvOS, and watchOS.
 
 Unlike other Markdown libraries, MMMarkdown implements an actual parser. It is not a port of the original Perl implementation and does not use regular expressions to transform the input into HTML. MMMarkdown tries to be efficient and minimize memory usage.