Add CMake minimum version required.

[g199209]

Interface Libraries feature is not available before CMake 3.0

If we add find_package(RapidJSON) in CMakeLists.txt and use old cmake to build the project, the cmake progress will failed:

CMake Error at CMakeLists.txt:xx (add_library):     
  Cannot find source file:   
         INTERFACE
  Tried extensions .c .C .c++ .cc .cpp .cxx .m .M .mm .h .hh .h++ .hm .hpp .hxx .in .txx     

This feature is only available after CMake 3.0, see this discussion for detail.

[g199209]

@tchernobog

[coveralls]

Coverage decreased (-0.1%) to 99.739% when pulling 1ce28f4 on g199209:master into f376690 on Tencent:master.

[coveralls]

Coverage decreased (-0.1%) to 99.739% when pulling 1ce28f4 on g199209:master into f376690 on Tencent:master.

[miloyip]

Thank you