Bump up version to 0.2.7

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124791220

Bump up version to 0.2.6

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124561124

Bump up version to 0.2.5

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123678175

Bump up version to 0.2.4

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122092925

Merge branch 'moe_writing_branch_from_4075ca967e5d2a78929eec99e6cc23a…

…14fde4790'

Bump up version to 0.2.2

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118731261

Update some str methods for recent SDK representation changes

Update Operation.str() to reflect that receivers is now an array
instead of a defaultdict.

Update Worker* objects to not output serialized combine_fn.

These problems surface only when logging at DEBUG level.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117727480

Small fixes in BigQuery snippets and wordcount example.