Add cargo fmt to CI and delete rustfmt.toml

This commit adds CI to run `cargo fmt` over Cargo itself as well as the internal
`crates-io` crate. This should switch Cargo to the "default style" (aka whatever
rustfmt spits out) and ensure that we keep it that way via CI. Hopefully this
won't be too much of a bother to keep up and running in CI as it should just be
a `cargo fmt` away!

.travis.yml

@@ -36,6 +36,9 @@ matrix:
         - cargo doc --no-deps
         - (cd src/doc && mdbook build --dest-dir ../../target/doc)
 
+    - before_script: rustup component add rustfmt-preview
+      script: cargo fmt -- --write-mode diff
+
   exclude:
     - rust: stable
 

rustfmt.toml

@@ -1 +0,0 @@
-disable_all_formatting = true