fbcode_builder: Update cmake for debian 8.6 os image
fbcode_builder: Update cmake for debian 8.6 os image

Summary: Debian 8.6 comes with a CMake version that is too old for folly. Add commands to update cmake.

Reviewed By: snarkmaster

Differential Revision: D6842797

fbshipit-source-id: 22f4c407f82a92c3fa90a5c2de334cf8b92e584b

facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,rklabs/folly,facebook/folly,rklabs/folly,facebook/folly,rklabs/folly