Manifest file strategy (updated)

A revision of the original #15, to work around not having write access on older PRs.

[x] Update tests for PHPUnit 6
[x] Address review points raised by @theofidry
[x] Check setter methods to other strategies for consistency (use setters vs constructor params)
[x] Implement additional features where warranted as noted in https://github.com/humbug/phar-updater/pull/15#issuecomment-222658921
[x] Update file hash checks to optionally use SHA-256 over SHA-1 (so we don't have to update them later!)
[x] Where relevant, migrate to use Composer\Semver
[x] Review test coverage and add where needed.
[x] Update README

humbug / phar-updater