Upgrade Go protobuf compiler to 1.2

The release notes: https://github.com/golang/protobuf/releases

V1.2

#605 jsonpb.Unmarshal correctly handles escaped strings for some of the well-known types

#627 jsonpb.Unmarshal correctly returns an error when trying to marshal a scalar enum into repeated enums.

#630 jsonpb.Unmarshal correctly handles string versions of numeric primitives

#644 jsonpb.Unmarshal correctly handles enum values in maps

#624 fixes a performance regression where nested maps led to quadratic behavior

#628 reverts strict UTF-8 validation for proto2 string fields

#660 makes UTF-8 validation errors for proto3 string fields non-fatal

V1.1

The serialization logic (for both Marshal and Unmarshal) has been optimized. Testing inside Google demonstrate that the new implementation is about 1.3x to 2.1x faster.
protocolbuffers/protobuf#272 Unknown fields in proto3 are now preserved.

#472 JSON marshal and unmarshal now check for proto2 required field.

#483 GetExtension returns the raw bytes if the ExtensionDesc does not have type information.

#499 Invalid UTF-8 in string types are rejected during marshal and unmarshal.

#511 Fields of bad wire types are treated as unknown fields instead of an error.

#544 The protoc-gen-go generator supports the paths=source_relative option.

#561 Unknown fields in map entries are now ignored instead of causing an unmarshal error.

#583 The "import public" feature is now implemented using Go type aliases.

Closes: gitaly#1497 (closed)