circle.yml: install new deps 'bison' and 'flex'
circle.yml: install new deps 'bison' and 'flex'

Since Linux 4.16, bison and flex are required for kernel builds.

https://github.com/torvalds/linux/commit/033dba2ec06c47a9fe1b190bc3281058fb20738d

kinvolk/stage1-builder