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
Make ga core collector lazy

This replaces lists and list comprehensions with generators in the ga
core collector. Combined with the changes in 628036e this should allow
the GA core collector to be much more memory efficient by allowing it to
fetch more pages from GA when it needs and send more chunks to backdrop
when it needs.

Note that if plugins are used all bets are off. I have not looked into
them much but the ones I have would not work.
Merge pull request #89 from alphagov/feature/streaming-ga-chunks

Make ga core collector lazy
BUGFIX: Remove length check on ga.send_data

Now that we're using generators we can't easily test whether there are
any documents without reading into the generator. It also doesn't give
very much value.

It may be useful in the future for the post method on DataSet to return
the number of documents that it sent.
Merge pull request #90 from alphagov/remove-length-check

BUGFIX: Remove length check on ga.send_data
Set default chunk size for GA collector to 100

We would like requests chunked up when talking to backdrop as not doing
so has been shown to bring it down. This sets the default to 100 records
so that we don't have to remember to set it explicitly in every config.
Merge pull request #91 from alphagov/chunk-ga-payloads

Set default chunk size for GA collector to 100