Use std::make_unique instead of our polyfill or naked new

The use we previously used the polyfill or naked new is that we
supported C++11, which did not yet have `std::make_unique`. However,
with the move to C++14 as the minimum, `std::make_unique` can be
expected to be always available.

src/catch2/catch_stream.cpp

@@ -108,7 +108,7 @@ namespace Catch {
             mutable std::ostream m_os;
         public:
             DebugOutStream()
-            :   m_streamBuf( new StreamBufImpl<OutputDebugWriter>() ),
+            :   m_streamBuf( std::make_unique<StreamBufImpl<OutputDebugWriter>>() ),
                 m_os( m_streamBuf.get() )
             {}