Fix bug where deriven Default impl causes panics

[dvdplm]

Without this fix this code is a broken triedb:

let mut db = MemoryDB::<KeccakHasher>::default();
let mut root = H256::new();
let mut tdb = TrieDBMut::new(&mut db, &mut root);
tdb.insert(b"not your pony").unwrap() // <–– blows up because the MemoryDB is not properly init'd

Version bump: 0.2.1

Closes #8

[ascjones]

👍