Use 3.16 as min cmake

.travis.yml

@@ -282,7 +282,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-8 && CXX=g++-8"
         - BUILD_TYPE=Debug
-        - CMAKE_EXE=/opt/local/cmake-3.9/bin/cmake
+        - CMAKE_EXE=/opt/local/cmake/bin/cmake
         - SKIP_TESTS=true
     # validator keys project as subproj of rippled
     - <<: *linux

src/ripple/protocol/STInteger.h

@@ -72,7 +72,7 @@ private:
     STBase*
     move(std::size_t n, void* buf) override;
 
-    friend class detail::STVar;
+    friend class ripple::detail::STVar;
 };
 
 using STUInt8 = STInteger<unsigned char>;