[skip ci] Change Milvus c++ standard to c++17 (#932)

* [skip ci]#668 - Update badge of README

* Fix README.md

* Fix README.md

* Merge remote-tracking branch 'upstream/master'

* #910 Change c++ standard to c++17

* Remove unused headers

* #910 Change Milvus c++ standard to c++17

* #910 Change Milvus c++ standard to c++17

install.md → INSTALL.md

@@ -13,6 +13,7 @@ If you encounter any problems/issues compiling Milvus from source, please refer
 
   If your operating system is not Ubuntu 18.04 or higher, we recommend you to pull a [docker image of Ubuntu 18.04](https://docs.docker.com/install/linux/docker-ce/ubuntu/) as your compilation environment.
   
+- GCC 7.0 or higher to support C++17
 - CMake 3.12 or higher
 
 ##### For GPU-enabled version, you will also need: