Add and apply AmbiguousRegexpLiteral rule

This is a very subtle behaviour: `match /attachment/i` could represent a
regular expression, but it could also represent a division like
`match / attachment / i`. So we need to make an exception to the usual
way we omit parenthesis in RSpec expectations.

.rubocop_basic.yml

@@ -117,6 +117,9 @@ Layout/TrailingBlankLines:
 Layout/TrailingWhitespace:
   Enabled: true
 
+Lint/AmbiguousRegexpLiteral:
+  Enabled: true
+
 Lint/LiteralAsCondition:
   Enabled: true