message: update cargo to rust master
  as suggested on discord, a sustainable method of selecting a version of
  `cargo` to use is whatever commit is being used by rust-lang/rust. the
  version selected in this commit uses the current version as of
  2019-05-13.
diff: @@ -177,7 +177,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
  [[package]]
  name = "cargo"
  version = "0.37.0"
  -source = "git+https://github.com/rust-lang/cargo.git#afd240e016947079e7e4aea240a742fc098d1a3f"
  +source = "git+https://github.com/rust-lang/cargo.git#759b6161a328db1d4863139e90875308ecd25a75"
  dependencies = [
  "atty 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "byteorder 1.2.6 (registry+https://github.com/rust-lang/crates.io-index)",
  @@ -352,7 +352,7 @@ dependencies = [
  [[package]]
  name = "crates-io"
  version = "0.25.0"
  -source = "git+https://github.com/rust-lang/cargo.git#afd240e016947079e7e4aea240a742fc098d1a3f"
  +source = "git+https://github.com/rust-lang/cargo.git#759b6161a328db1d4863139e90875308ecd25a75"
  dependencies = [
  "curl 0.4.21 (registry+https://github.com/rust-lang/crates.io-index)",
  "failure 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)",
  