ci: revert back to using gcc 4.9
ci: revert back to using gcc 4.9

This may solve an issue with running prebuilt binaries on CentOS 7.

Refs: https://github.com/addaleax/lzma-native/issues/88

addaleax/lzma-native,addaleax/lzma-native,addaleax/lzma-native,addaleax/lzma-native