Switch to Rust 1.31 as minimum supported version

Too many dependencies switched and we can't really keep everything at an
older version.

Commented out for now until stable becomes 1.32.
2 jobs for master in 6 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #67570
allowed to fail
test nightly

00:06:24

passed #67569
test stable

00:06:52