Fix CI failures by resolving RuboCop offenses

[garettarrowood]

Resolves RuboCop failures present in travis build failure on master. These changes make rubocop complete successfully. Before starting, these were the 5 reported offenses:

Offenses:

lib/modis.rb:31:3: W: Lint/DuplicateMethods: Method Modis.connection_pool is defined at both lib/modis.rb:23 and lib/modis.rb:31.
  def self.connection_pool
  ^^^
spec/spec_helper.rb:6:5: C: Style/MixinUsage: include is used at the top level. Use inside class or module.
    include SimpleCovHelper
    ^^^^^^^^^^^^^^^^^^^^^^^
lib/modis/persistence.rb:196:1: W: Lint/MissingCopEnableDirective: Re-enable Metrics/AbcSize cop with # rubocop:enable after disabling it.
    # rubocop:disable Metrics/AbcSize, Metrics/PerceivedComplexity
^
lib/modis/persistence.rb:237:28: C: Performance/HashEachMethods: Use each_key instead of each.
        changed_attributes.each do |k, _|
                           ^^^^
lib/modis/attribute.rb:56:39: C: Style/EvalWithLocation: Use __LINE__ + 1 instead of __LINE__, as they are used by backtraces.
        class_eval <<-RUBY, __FILE__, __LINE__

[Tonkpils]

👍 Thank you so much for taking care of these fixes!