Use #rm_rf rather than #remove_entry_secure

On some systems, `FileUtils#remove_entry_secure` fails to work because
it fails the [TOCTTOU vulnerability check](1), even for `/tmp`. In the
case of nanoc, the vulnerability check has little use, since it deletes
the directories that it creates itself, so using `FileUtils#rm_rf`
instead of `FileUtils#remove_entry_secure` is fine.

Also see nanoc/nanoc#465

[1]: http://www.ruby-doc.org/stdlib-2.1.2/libdoc/fileutils/rdoc/FileUtils.html#method-c-remove_entry_secure
Prefix unused arguments with an underscore
Disable some cops (for now)
Display cop names by default
Prefix unused method arguments with an underscore
Fix LineEndConcatenation offenses
Update Gemfile.lock
fix SingleSpaceBeforeFirstArg offenses
Fix StringLiterals offenses
Fix RedundantSelf offenses
Fix SpaceBeforeSemicolon offenses
Fix PercentLiteralDelimiters offenses
Fix SelfAssignment offenses
Fix VariableName offenses
Fix SpaceAroundEqualsInParameterDefault offenses
Fix Next offenses
Fix PredicateName and DoubleNegation offenses
Fix RedundantBegin offenses
Update Rubocop config
Use unless where it makes sense