message: c++14 in installation documentation
diff: * Examples of GUDHI headers inclusion can be found in \ref utilities.
  *
  * \section compiling Compiling
  - * The library uses c++11 and requires <a target="_blank" href="http://www.boost.org/">Boost</a> &ge; 1.56.0
  + * The library uses c++14 and requires <a target="_blank" href="http://www.boost.org/">Boost</a> &ge; 1.56.0
  * and <a target="_blank" href="https://www.cmake.org/">CMake</a> &ge; 3.1.
  * It is a multi-platform library and compiles on Linux, Mac OSX and Visual Studio 2015.
  *
  