cmake 2.8 build failed

[moonming]

https://github.com/xpol/lua-rapidjson/blob/master/CMakeLists.txt#L68
the target_compile_features() command was added in cmake 3.1, so https://github.com/xpol/lua-rapidjson/blob/master/CMakeLists.txt#L1 cmake 2.8 can not build.
maybe we can remove target_compile_features?

[xpol]

I have try to support cmake 2.8, try version 0.6.1.