Bump protobuf from 1.4.2 to 2.6.0

[dependabot[bot]]

Bumps protobuf from 1.4.2 to 2.6.0.

Changelog

Sourced from protobuf's changelog.

[2.6.0] - 2019-05-19

• lite_runtime rust-protobuf option
• Fix OOM on malformed input
• Minimum supported Rust version is 1.26
• Implement Hash for UnknownFields

[2.5.0] - 2019-04-15

• generate_accessors options to disable generation of accessor functions (except getters).
• Default is now implented for all &MyMessage types
• Debug is now implemented for oneof enums

[2.4.2] - 2019-03-29

• Fix well-known types codegen
• More extension types are supported now

[2.3.1] - 2019-03-05

• Fix codegen when syntax = "proto2" and with-bytes option and string field with default value

[2.3.0] - 2019-01-30

• Default is implemented for enums even in proto2
• Fix codegen where map value is an enum

[2.2.5] - 2019-01-20

• Escape extension field names

[2.2.4] - 2019-01-13

• Replace tempdir dependency with tempfile
• Fix serialization of signed int map keys or values

[2.2.2] - 2018-12-29

• Fix codegen on Windows

[2.2.1] - 2018-12-25

• Fix panic with oneof and bytes codegen

[2.2.0] - 2018-11-17

... (truncated)

Commits

• e506e96 Bump version
• 8235552 Implement Hash for UnknownFields
• 71f09ae Change minimum supported Rust version to 1.26
• f32ac1c Fuzz testing with Read trait and fix OOM on incorrect input
• de13d72 add missing codegen rules
• acdc94a *: support add lite-runtime customization options
• 8b3ec6f Bump version
• abcc3db build before regenerate
• f9ed7a5 Regenerate code after Debug implemented for oneof enums
• 859d7f5 Derive Debug for oneof enums
• Additional commits viewable in compare view

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