Update rustfmt to 0.3.6

Fixes a possible bug from 0.3.5
(rust-lang/rust#47454 (comment))
and prepares the RLS repo to be updated in Rust tree.

Author: Xanewok

Cargo.lock

@@ -23,7 +23,7 @@ rls-data = { version = "0.14", features = ["serialize-serde"] }
 rls-rustc = "0.1"
 rls-span = { version = "0.4", features = ["serialize-serde"] }
 rls-vfs = { version = "0.4", features = ["racer-impls"] }
-rustfmt-nightly = { version = "0.3.5", optional = true }
+rustfmt-nightly = { version = "0.3.6", optional = true }
 serde = "1.0"
 serde_json = "1.0"
 serde_derive = "1.0"