CMake tweaks
  - I think CMake 3.1 is enough for now.
  - Ubuntu works too.

BUG=skia:4269

Review URL: https://codereview.chromium.org/1323093002
diff --git a/cmake/CMakeLists.txt b/cmake/CMakeLists.txt
index ea98cd1..e83a2b0 100644
--- a/cmake/CMakeLists.txt
+++ b/cmake/CMakeLists.txt
@@ -1,5 +1,5 @@
 # Boilerplate.
-cmake_minimum_required (VERSION 3.3)   # The latest version as I write this.  May be too strict.
+cmake_minimum_required (VERSION 3.1)   # First version with CMAKE_CXX_STANDARD.
 project (skimake)
 set (CMAKE_CXX_STANDARD 11)
 
diff --git a/cmake/README.md b/cmake/README.md
index 36c1c3d..9fd84a7 100644
--- a/cmake/README.md
+++ b/cmake/README.md
@@ -20,7 +20,7 @@
 
 Currently maybe-kinda-working platforms
 ---------------------------------------
-  - x86-64 Mac OS X built with Clang (i.e. mtklein's laptop).
+  - x86-64 Mac OS X, Ubuntu 15.04
 
 Caveats
 -------