Update protobuf requirement from 3.6.1 to 3.7.1

[dependabot-preview]

Updates the requirements on protobuf to permit the latest version.

Release notes

Sourced from protobuf's releases.

Protocol Buffers v3.7.1

C++

• Avoid linking against libatomic in prebuilt protoc binaries (#5875)
• Avoid marking generated C++ messages as final, though we will do this in a future release (#5928)
• Miscellaneous build fixes

JavaScript

• Fixed redefinition of global variable f (#5932)

Ruby

• Replace strptime with custom implementation (#5906)
• Fixed the bug that bytes fields cannot be larger than 16000 bytes (#5924)
• Miscellaneous bug fixes

PHP

• Replace strptime with custom implementation (#5906)
• Fixed the bug that bytes fields cannot be larger than 16000 bytes (#5924)

Commits

• 6973c3a Update version number to 3.7.1 (#5947)
• 0770795 Fixed CI failures with Ruby 2.3 on OSX (#5948)
• 34b6499 Avoid linking against libatomic when building protoc (#5922)
• ba42cb5 Allow bytes field to be longer than 16000 bytes (#5924)
• 02cf61a Updated NPM version to 3.7.1
• ec0d47e No define strtoll/strtoull for VS2013 and above
• b3c132f Avoid marking generated C++ messages as final for now (#5928)
• 059851f Remove unused import from message.
• e2461b5 Revert "Remove unused f expression from jscode gen. (#5573)"
• e6779e9 Added io_win32.h to header list in CMake config
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #361.