Update minimum rust version to 1.20.0
Update minimum rust version to 1.20.0

This is indirectly required due to use of associated constants in the
upstream bitflags crate.

Signed-off-by: Paul Osborne <b6afffd6c1a00645fdd04b09edf63d034a949a39@gmail.com>

rust-embedded/rust-sysfs-gpio