Use git source for derive_builder for now, for attrs feature

We are going to want to specify custom attributes on fields of the
builder struct.  This feature was missing from derive_builder.

This commitid is the current head of my MR branch
  https://github.com/colin-kiegel/rust-derive-builder/pull/237
  https://github.com/ijackson/rust-derive-builder/tree/builder-field-attrs
Using the commitid prevents surprises if that branch is updated.

We will require this newer version of derive_builder.  The version
will need to be bumped again later, assuming the upstream MR is merged
and upstream do a release containing the needed changes.