Update ubuntu-12.04-detailed.md

- add ppa to install latest version of cmake. old version which came with ubuntu-12.04 has a problem with valgrind.
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doc/make/ubuntu-12.04-detailed.md

@@ -6,6 +6,7 @@ Preparing working environment on Ubuntu 12.04
     sudo apt-get install git
     sudo apt-get install libgmp-dev
     sudo apt-get install libmpfr-dev
+    sudo add-apt-repository ppa:kalakris/cmake -y
     sudo apt-get install cmake
 
     sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y