Release 3.1.0

• Remove support for Ruby < 2.4.
• Make sure buffer encoding set using format or passed option.
• Fix transaction check and enable local running of spec.
• Rename instance variables used in specs to be localized so as not to confuse with the same variable that could be named in the surrounding context.
• Allow readers to define more default prefixes.
• Allow dump format to be specified in be_equivalent_graph.
• Add check for default and user-specified User-Agent.
• Fix write matcher to handle exceptions in block.
• Don't override Array.inspect, and use awesome_print in produce matcher.
• Be explicit when argument is a hash and not keyword arguments.