Cargo.toml: Bump license-exprs to 1.4.0

And then update Cargo.lock with:

  $ cargo build

This bumps us from the SPDX License List 2.4 to 3.1.  Some 2.4
identifiers have since been deprecated by SPDX, but none of them have
been removed, so it should be safe to deploy this before bumping Cargo
and its docs (which mention 2.4 since [1]).

[1]: rust-lang/cargo#4898

Author: wking

Cargo.lock

@@ -42,7 +42,7 @@ log = "0.3"
 env_logger = "0.5"
 hex = "0.3"
 htmlescape = "0.3.1"
-license-exprs = "^1.3"
+license-exprs = "^1.4"
 dotenv = "0.11.0"
 toml = "0.4"
 diesel = { version = "1.3.0", features = ["postgres", "serde_json", "chrono", "r2d2"] }