Fix symbol(s) not found error.

[yanzhiwei147]

Fix #255 issue.

[houndci-bot]

Some files could not be reviewed due to errors:

.rubocop.yml: Lint/DefEndAlignment has the wrong namespace - should be Layout

.rubocop.yml: Lint/DefEndAlignment has the wrong namespace - should be Layout
.rubocop.yml: Lint/EndAlignment has the wrong namespace - should be Layout
.rubocop.yml: Lint/Eval has the wrong namespace - should be Security
.rubocop.yml: Style/AlignHash has the wrong namespace - should be Layout
.rubocop.yml: Style/AlignParameters has the wrong namespace - should be Layout
.rubocop.yml: Style/CaseIndentation has the wrong namespace - should be Layout
.rubocop.yml: Style/ClosingParenthesisIndentation has the wrong namespace - should be Layout
.rubocop.yml: Style/CommentIndentation has the wrong namespace - should be Layout
.rubocop.yml: Style/EmptyLineBetweenDefs has the wrong namespace - should be Layout
.rubocop.yml: Style/EmptyLines has the wrong namespace - should be Layout
.rubocop.yml: Style/EmptyLinesAroundBlockBody has the wrong namespace - should be Layout
.rubocop.yml: Style/EmptyLinesAroundClassBody has the wrong namespace - should be Layout
.rubocop.yml: Style/EmptyLinesAroundModuleBody has the wrong namespace - should be Layout
.rubocop.yml: Style/ExtraSpacing has the wrong namespace - should be Layout
.rubocop.yml: Style/FirstParameterIndentation has the wrong namespace - should be Layout
.rubocop.yml: Style/IndentationWidth has the wrong namespace - should be Layout
.rubocop.yml: Style/LeadingCommentSpace has the wrong namespace - should be Layout
.rubocop.yml: Style/MultilineOperationIndentation has the wrong namespace - should be Layout
.rubocop.yml: Style/SpaceAfterComma has the wrong namespace - should be Layout
.rubocop.yml: Style/SpaceAroundEqualsInParameterDefault has the wrong namespace - should be Layout
.rubocop.yml: Style/SpaceAroundOperators has the wrong namespace - should be Layout
.rubocop.yml: Style/SpaceInsideBlockBraces has the wrong namespace - should be Layout
.rubocop.yml: Style/SpaceInsideHashLiteralBraces has the wrong namespace - should be Layout
.rubocop.yml: Style/SpaceInsideParens has the wrong namespace - should be Layout
.rubocop.yml: Style/TrailingBlankLines has the wrong namespace - should be Layout
.rubocop.yml: Style/TrailingWhitespace has the wrong namespace - should be Layout
Error: The `Style/TrailingComma` cop no longer exists. Please use `Style/TrailingCommaInArguments`, `Style/TrailingCommaInArrayLiteral`, and/or `Style/TrailingCommaInHashLiteral` instead.
(obsolete configuration found in .rubocop.yml, please update it)