Cargo.toml: Bump to v1.4.0

[wking]

Changes since v1.3.0:

• Updated SPDX License List to 3.1 (Add UPL-1.0 identifier from https://spdx.org/licenses/ #9, Add examples/fetch.rs to fetch spdx.org/licenses and format it as src/spdx.rs #11, README: Document our license-list version, parens issue, and license #17, src/spdx.rs: Bump to SDPX License List 3.1 #19, Move fetch-license-list-from-spdx to separate crate #24).
• Updated SPDX License Expression reference from 2.0 to 2.1 (README.md: update the reference to SPDX #8).
• Document our license-list version, parens issue, and license (Cargo.toml: Replace '/' with 'OR' in 'license' #16, README: Document our license-list version, parens issue, and license #17).
• Add Travis CI configuration (Continuous Integration #22).
• Add additional test cases (Extend test cases #25).
• .mailmap: Consolidate authors (.mailmap: Consolidate authors #26).

I'll give this at least a week to cook, and then merge, tag, and push a release to crates.io.

Fixes #18.
Fixes #27.

[wking]

I've tagged v1.4.0 and pushed it to crates.io.