Replace Boost usage with C++11 features

- 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

src/ptr_vector.h

@@ -16,33 +16,35 @@
 
 namespace YAML {
 
+// TODO: This class is no longer needed
 template <typename T>
 class ptr_vector : private YAML::noncopyable {
  public:
   ptr_vector() {}
-  ~ptr_vector() { clear(); }
 
   void clear() {
-    for (std::size_t i = 0; i < m_data.size(); i++)
-      delete m_data[i];
     m_data.clear();
   }
 
   std::size_t size() const { return m_data.size(); }
   bool empty() const { return m_data.empty(); }
 
-  void push_back(std::auto_ptr<T> t) {
-    m_data.push_back(NULL);
-    m_data.back() = t.release();
+  void push_back(std::unique_ptr<T>&& t) {
+    m_data.push_back(std::move(t));
   }
   T& operator[](std::size_t i) { return *m_data[i]; }
   const T& operator[](std::size_t i) const { return *m_data[i]; }
 
-  T& back() { return *m_data.back(); }
-  const T& back() const { return *m_data.back(); }
+  T& back() {
+      return *(m_data.back().get());
+  }
+
+  const T& back() const {
+      return *(m_data.back().get());
+  }
 
  private:
-  std::vector<T*> m_data;
+   std::vector<std::unique_ptr<T>> m_data;
 };
 }