update required cmake version to 2.8.10

Change-Id: Ibba0271efa86e1b1af97f2a08b73677dfd236b7a
Reviewed-on: https://boringssl-review.googlesource.com/12986
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

BUILDING.md

@@ -2,7 +2,7 @@
 
 ## Build Prerequisites
 
-  * [CMake](https://cmake.org/download/) 2.8.8 or later is required.
+  * [CMake](https://cmake.org/download/) 2.8.10 or later is required.
 
   * Perl 5.6.1 or later is required. On Windows,
     [Active State Perl](http://www.activestate.com/activeperl/) has been