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.

examples/300-Gen-OwnGenerator.cpp

@@ -40,7 +40,9 @@ int const& RandomIntGenerator::get() const {
 // Notice that it returns an instance of GeneratorWrapper<int>, which
 // is a value-wrapper around std::unique_ptr<IGenerator<int>>.
 Catch::Generators::GeneratorWrapper<int> random(int low, int high) {
-    return Catch::Generators::GeneratorWrapper<int>(std::unique_ptr<Catch::Generators::IGenerator<int>>(new RandomIntGenerator(low, high)));
+    return Catch::Generators::GeneratorWrapper<int>(
+        std::make_unique<RandomIntGenerator>(low, high)
+    );
 }
 
 // The two sections in this test case are equivalent, but the first one

examples/301-Gen-MapTypeConversion.cpp

@@ -38,9 +38,7 @@ std::string const& LineGenerator::get() const {
 // is a value-wrapper around std::unique_ptr<IGenerator<std::string>>.
 Catch::Generators::GeneratorWrapper<std::string> lines(std::string /* ignored for example */) {
     return Catch::Generators::GeneratorWrapper<std::string>(
-        std::unique_ptr<Catch::Generators::IGenerator<std::string>>(
-            new LineGenerator()
-        )
+        std::make_unique<LineGenerator>()
     );
 }