Add missing LICENSE file in published version-ranges crates

[musicinmybrain]

Adds a symbolic link to the top-level LICENSE file, causing it to be included as a regular file in published version-ranges crates.

Before:

$ cargo publish --dry-run
$ tar -tvzf ../target/package/version-ranges-0.1.0.crate
-rw-r--r-- 0/0             108 1969-12-31 19:00 version-ranges-0.1.0/.cargo_vcs_info.json
-rw-r--r-- 0/0            1299 1969-12-31 19:00 version-ranges-0.1.0/Cargo.toml
-rw-r--r-- 0/0             600 2006-07-23 21:21 version-ranges-0.1.0/Cargo.toml.orig
-rw-r--r-- 0/0           46607 2006-07-23 21:21 version-ranges-0.1.0/src/lib.rs

After:

$ cargo publish --dry-run
$ tar -tvzf ../target/package/version-ranges-0.1.0.crate
-rw-r--r-- 0/0             108 1969-12-31 19:00 version-ranges-0.1.0/.cargo_vcs_info.json
-rw-r--r-- 0/0            1299 1969-12-31 19:00 version-ranges-0.1.0/Cargo.toml
-rw-r--r-- 0/0             600 2006-07-23 21:21 version-ranges-0.1.0/Cargo.toml.orig
-rw-r--r-- 0/0           16725 2006-07-23 21:21 version-ranges-0.1.0/LICENSE
-rw-r--r-- 0/0           46607 2006-07-23 21:21 version-ranges-0.1.0/src/lib.rs