Cargo.toml: Bump to v1.4.0

Changes since v1.3.0:

* Updated SPDX License List to 3.1 (ehuss#9, ehuss#11, ehuss#17, ehuss#19, ehuss#24).
* Updated SPDX License Expression reference from 2.0 to 2.1 (ehuss#8).
* Document our license-list version, parens issue, and license (ehuss#16,
  ehuss#17).
* Add Travis CI configuration (ehuss#22).
* Add additional test cases (ehuss#25).
* .mailmap: Consolidate authors (ehuss#26).

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "license-exprs"
-version = "1.3.0"
+version = "1.4.0"
 authors = ["Without Boats <[email protected]>"]
 license = "Apache-2.0 OR MIT"
 description = "Validate SPDX 2.1 license expressions using SPDX License List 3.1 identifiers."