Upgrade performanceplatform-client to 0.2.1

We need to use the latest client library to enable the ability to chunk
our posts. The JsonEncoder has moved in this release so I have changed
the import to reflect its new location.
Pass chunk size option through to data set post

We want to be able to define a chunk size for our posts so that we don't
try and submit too many records in one go.
Merge pull request #88 from alphagov/feature/chunk-ga-collector

Feature/chunk ga collector