Redo build matrix on .travis for C++14 and up

* Use Xenial as the base distribution
* Remove C++11 builds
* Add a lot more C++14 builds
* Add some C++17 builds
* Include newer versions of Clang and GCC

examples/301-Gen-MapTypeConversion.cpp

@@ -22,15 +22,17 @@ public:
         }
     }
 
-    std::string const& get() const override {
-        return m_line;
-    }
+    std::string const& get() const override;
     
     bool next() override {
         return !!std::getline(m_stream, m_line);
     }
 };
 
+std::string const& LineGenerator::get() const {
+    return m_line;
+}
+
 // This helper function provides a nicer UX when instantiating the generator
 // Notice that it returns an instance of GeneratorWrapper<std::string>, which
 // is a value-wrapper around std::unique_ptr<IGenerator<std::string>>.