* Be specific about the `BitVec` generic arguments

Currently we use the default generic arguments for `BitVec`. This means
we use `BigEndian` and `u8`. These default values are not stable, with
`0.17` of the `BitVec` crate this changes. To make sure we don't break
anything in the future, make sure we explictly set the generics.

* Update `spec_version`