Fix RuboCop offenses

[y-yagi]

This PR fixes following offenses.

Offenses:

lograge.gemspec:31:5: C: [Corrected] Gemspec/AddRuntimeDependency: Use add_dependency instead of add_runtime_dependency.
  s.add_runtime_dependency 'actionpack',    '>= 4'
    ^^^^^^^^^^^^^^^^^^^^^^
lograge.gemspec:32:5: C: [Corrected] Gemspec/AddRuntimeDependency: Use add_dependency instead of add_runtime_dependency.
  s.add_runtime_dependency 'activesupport', '>= 4'
    ^^^^^^^^^^^^^^^^^^^^^^
lograge.gemspec:33:5: C: [Corrected] Gemspec/AddRuntimeDependency: Use add_dependency instead of add_runtime_dependency.
  s.add_runtime_dependency 'railties', '>= 4'
    ^^^^^^^^^^^^^^^^^^^^^^
lograge.gemspec:34:5: C: [Corrected] Gemspec/AddRuntimeDependency: Use add_dependency instead of add_runtime_dependency.
  s.add_runtime_dependency 'request_store', '~> 1.0'