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
 

spec/features/admin/emails/emails_download_spec.rb

@@ -33,8 +33,8 @@ describe "Admin download user emails" do
       end
 
       header = page.response_headers["Content-Disposition"]
-      expect(header).to match /^attachment/
-      expect(header).to match /filename="Administrators.csv"$/
+      expect(header).to match(/^attachment/)
+      expect(header).to match(/filename="Administrators.csv"$/)
 
       file_contents = page.body.split(",")
       expect(file_contents).to match_array ["admin_news1@consul.dev", "admin_news2@consul.dev"]