v6.9.1
* Fixed warning "unused variable" with GCC 4.4 (issue #912) * Fixed warning "cast increases required alignment" (issue #914) * Fixed warning "conversion may alter value" (issue #914) * Fixed naming conflict with "CAPACITY" (issue #839) * Muted warning "will change in GCC 7.1" (issue #914) * Added a clear error message for `StaticJsonBuffer` and `DynamicJsonBuffer` * Marked ArduinoJson.h as a "system header"