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/231-Cfg-OutputStreams.cpp

@@ -10,11 +10,12 @@
 #define CATCH_CONFIG_MAIN
 #include <catch2/catch.hpp>
 
+
 class out_buff : public std::stringbuf {
     std::FILE* m_stream;
 public:
-    out_buff(std::FILE* stream) :m_stream(stream) {}
-    ~out_buff() { pubsync(); }
+    out_buff(std::FILE* stream):m_stream(stream) {}
+    ~out_buff();
     int sync() {
         int ret = 0;
         for (unsigned char c : str()) {
@@ -29,6 +30,12 @@ public:
     }
 };
 
+out_buff::~out_buff() { pubsync(); }
+
+#if defined(__clang__)
+#pragma clang diagnostic ignored "-Wexit-time-destructors" // static variables in cout/cerr/clog
+#endif
+
 namespace Catch {
     std::ostream& cout() {
         static std::ostream ret(new out_buff(stdout));