electrum: Unify protobuf dependencies

Since 9c57f3b5

 bumped the protobuf
version because the new upstream requires it, electrum now gets
protobuf3_0 *and* protobuf3_2 instead of just one version.

This leads to the following build errer:

Found duplicated packages in closure for dependency 'protobuf':
  protobuf 3.0.2 (...-python2.7-protobuf-3.0.2/lib/python2.7/site-packages)
  protobuf 3.2.0 (...-python2.7-protobuf-3.2.0/lib/python2.7/site-packages)

Using protobuf3_2 for keepkey and electrum fixes the build.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @np