SwiftProtobuf 1.5.0 Release

Complete support for Swift 5.

• Swift5: Fix warnings about loss of precision (#844)
• Drop the support for Swift <4.0. (#847)
• Swift5.0: Change Data(bytes:) to Data(_:). (#848)
• Fix build command to work also with Swift 5. (#852)
• Swift 5.0: Switch a stray Data(bytes:) to Data(_:). (#854)
• Swift 5: adopt new Data.withUnsafeBytes API. (#843)

Update some internals:

• Update some FileIo calls to FileHandle. (#845)
• Remove printToFd logic from FileIo. (#846)

Add some features to the library:

• TextFormat conformance: Add option to suppress unknown field printing. (#850)
• Adding JSON encoding option: Use proto field names instead of lowerCamelCase names. (#856)