GitLab

 - Adds 'std=c++11' compiler flags
 - Replaces boost::type_traits with std::type_traits
 - Replaces boost::shared_ptr with std::shared_ptr
 - Replaces std::auto_ptr with std::unique_ptr
 - Replaces raw pointers with std::unique_ptr in ptr_vector, ptr_stack, and SettingChanges
 - Replaces boost::noncopyable with deleted copy and assignment operators
 - Replaces boost::next with std::next
 - Replaces boost::enable_if with std::enable_if
 - Replaces boost::is_convertible with std::is_convertible
 - Replaces ptrdiff_t with std::ptrdiff_t
 - Replaces boost::iterator_facade and boost::iterator_adaptor with std::iterator, borrowing the 'proxy reference' technique from boost
 - Removes Boost dependency from CMakeLists
 - Formats changed files using clang-format

Applied the patch given in jbeder/yaml-cpp#200 with the correct code
style.