build(deps): Depend on heck >=0.4, <0.6 (#508)

Change the dependency on heck from 0.5 to >=0.4, <0.6.

Author: govinda-kamath

cargo-martian/Cargo.toml

@@ -12,4 +12,4 @@ docopt = "1.0"
 serde = { version = "1.0", features = ['derive'] }
 serde_json = "1.0"
 strfmt = ">=0.1.6, <0.3"
-heck = "0.5"
+heck = ">=0.4, <0.6"

martian/Cargo.toml

@@ -13,7 +13,7 @@ license = "MIT"
 anyhow = { version = "1", features = ["backtrace"] }
 backtrace = "0.3"
 fern = ">=0.5, <0.7"
-heck = "0.5"
+heck = ">=0.4, <0.6"
 log = "0.4"
 rayon = { version = "1", optional = true }
 rustc_version = ">=0.3, <0.5"