bump minimum supported OS X version to 10.7

Closes rust-lang#11927
Closes rust-lang#10842
Closes rust-lang#2024

Author: thestinger

README.md

@@ -73,7 +73,7 @@ Snapshot binaries are currently built and tested on several platforms:
 
 * Windows (7, Server 2008 R2), x86 only
 * Linux (various distributions), x86 and x86-64
-* OSX 10.6 ("Snow Leopard") or greater, x86 and x86-64
+* OSX 10.7 (Lion) or greater, x86 and x86-64
 
 You may find that other platforms work, but these are our officially
 supported build environments that are most likely to work.

doc/tutorial.md

@@ -74,7 +74,7 @@ Snapshot binaries are currently built and tested on several platforms:
 
 * Windows (7, Server 2008 R2), x86 only
 * Linux (various distributions), x86 and x86-64
-* OSX 10.6 ("Snow Leopard") or greater, x86 and x86-64
+* OSX 10.7 (Lion) or greater, x86 and x86-64
 
 You may find that other platforms work, but these are our "tier 1"
 supported build environments that are most likely to work.