Exclude tests from publication

[nexdrew]

This came up recently in yargs/yargs#468 (comment).

Would be nice to trim the package size a bit by excluding tests, and I prefer whitelisting files in package.json over blacklisting files in .npmignore, but I can be persuaded otherwise.

Here is the result of npm pack before and after this change:

# BEFORE
x package/package.json
x package/.npmignore
x package/README.md
x package/LICENSE
x package/semver.js
x package/bin/semver
x package/.travis.yml
x package/range.bnf
x package/test/big-numbers.js
x package/test/clean.js
x package/test/gtr.js
x package/test/index.js
x package/test/ltr.js
x package/test/major-minor-patch.js

# AFTER
x package/package.json
x package/README.md
x package/LICENSE
x package/semver.js
x package/bin/semver
x package/range.bnf