Does not build on nightly

[janderholm]

Example does not build on current nightly.

error[E0437]: type `Err` is not a member of trait `convert::TryFrom`
  --> src/packet.rs:72:5
   |
72 |     type Err = Error;
   |     ^^^^^^^^^^^^^^^^^ not a member of trait `convert::TryFrom`

error[E0046]: not all trait items implemented, missing: `Error`
   --> src/packet.rs:71:1
    |
71  | / impl<T: ReadBytesExt> convert::TryFrom<T> for Packet {
72  | |     type Err = Error;
73  | |     fn try_from(mut rdr: T) -> Result<Packet> {
74  | |         let li_vn_mode = rdr.read_u8()?;
...   |
116 | |     }
117 | | }
    | |_^ missing `Error` in implementation
    |
    = note: `Error` from trait: `type Error;`

error: aborting due to previous error

error: Could not compile `ntp`.

[JeffBelgum]

Fixed by #9