srcDir can no longer contain a leading slash

[Gaurav0]

In broccoli-funnel 2.02, the following worked:

      let node = new Funnel('.', {
        srcDir: '/',
        destDir: 'foo',
      });

However, in 3.0.2, it fails with:

Relative path is expected, path / is an absolute path. inputPath gets prefixed to the reltivePath provided.

If this change is intended, it should be documented in changelog as a breaking change.

[Gaurav0]

Fixed by #128