Enable Style/RedundantReturn cop

Description of the proposal

• Enables Style/RedundantReturn cop
• Fixes existing Style/RedundantReturn offenses

# bad
def test
  return something
end

# bad
def test
  one
  two
  three
  return something
end

# good
def test
  return something if something_else
end

# good
def test
  if x
  elsif y
  else
  end
end