Upgrade toolchain used for running hooks to v3.1 (#59)

This resolves patch_rm2fb failing to install patchelf.

Author: palfrey

toltec/hooks/strip.py

@@ -20,7 +20,7 @@
 logger = logging.getLogger(__name__)
 
 MOUNT_SRC = "/src"
-TOOLCHAIN = "toolchain:v1.3.1"
+TOOLCHAIN = "toolchain:v3.1"
 
 
 def walk_elfs(src_dir: str, for_each: Callable) -> None: