Rollback bindgen to 0.31 release

The 0.42 release of bindgen was failing in the docker rust image. All
versions between 0.31 and 0.42 failed with other errors, so this is the only
version that works on both macOS and docker.

cargo test passes

Cargo.toml

@@ -14,5 +14,5 @@ build = "build.rs"
 libc = ">=0.2"
 
 [build-dependencies]
-bindgen = "0.42"
+bindgen = "0.31"
 pkg-config = "0.3"