Bump version number to beta-3.1

Merge pull request protocolbuffers#1540 from pherl/changelog

Update changes for lite

Merge pull request protocolbuffers#1510 from thomasvl/nonnull

Declare an init and avoid passing NULL to initWithValue:count:

Protocol Buffers v3.0.0-beta-2

v3.0.0-beta-1.1 for python.

Change-Id: I5b1506385849b3b352d9717f7ffbc26237c9fac3

Merge pull request protocolbuffers#915 from pherl/master

add default_runtime attribute to cc|py rules.

Merge pull request protocolbuffers#796 from jcanizales/add-missing-comma

Fix podspec by adding missing comma

Protocol Buffers v3.0.0-alpha-4

Merge pull request protocolbuffers#759 from TeBoring/beta-1-2

Fix Protobuf.podspec

bump the runtime version to 3.1.

This matches the protoc artifacts.

Change-Id: Iad8ffed6d0c61b79893a36ccafb420e79aca8d14