Maintenance

[jeffreyc]

• Bumping .ruby-version to 2.4.1
• Updating the Ruby versions in the Travis matrix and adding 2.4.1
• Bumping RuboCop
  • Disabling Layout/IndentHeredoc; it wasn't detecting the indentation correctly
  • Disabling Metrics/BlockLength for specs; specs should probably be longer than 25 lines
  • Disabling Security/YAMLLoad; YAML.safe_load is not present in Ruby 2.0
  • Disabling Style/FrozenStringLiteralComment
  • Assorted fixes for cops added since 0.40
• Bumping WebMock
• Not bumping Rake; bumping Rake requires bumping RSpec, which is a breaking change