message: Pin the abscissa version to 0.6.0-beta.1
  Unpinned, a `cargo update` "updates" it to 0.6.0-pre.2, which is later
  by semver, but not by the pre-release sequence used by abscissa.
diff: @@ -71,10 +71,10 @@ version = "=0.23.1"
  features = ["unstable"]
  [dependencies.abscissa_core]
  -version = "0.6.0-beta.1"
  +version = "=0.6.0-beta.1"
  features = ["options"]
  [dev-dependencies]
  -abscissa_core = { version = "0.6.0-beta.1", features = ["testing"] }
  +abscissa_core = { version = "=0.6.0-beta.1", features = ["testing"] }
  once_cell = "1.8"
  regex = "1.5"
  