message: Update slog-term, slog-async, slog-json
diff: @@ -30,9 +30,9 @@ gnuplot = { version = "0.0.37", optional = true }
  serde = { version = "1.0", features = ["derive", "rc"], optional = true }
  serde_json = { version = "1.0", optional = true }
  slog = { version = "2.4.1", optional = true }
  -slog-term = { version = "2.4.0", optional = true }
  -slog-async = { version = "2.3.0", optional = true }
  -slog-json = { version = "2.3.0", optional = true }
  +slog-term = { version = "2.8.1", optional = true }
  +slog-async = { version = "2.7.0", optional = true }
  +slog-json = { version = "2.5.0", optional = true }
  [dev-dependencies]
  approx = "0.5.0"
  