Fix a new rubocop rule introduced by a new version of rubocop

Since this is a gem it is a best practice to not include a Gemfile.lock, however
that also means that travis will download always the last versions of all gems.

In the last Travis run, the version of Rubocop was updated, throwing an error in
a rule that we may not neccessarily want to adopt, hence I am excluding it just
for the file that triggers it.

Relevant Rubocop issue:
  rubocop/rubocop#3438

.rubocop.yml

@@ -19,3 +19,7 @@ Metrics/LineLength:
 
 Style/RegexpLiteral:
   EnforcedStyle: percent_r
+
+Style/FormatStringToken:
+  Exclude:
+    - 'lib/donation_system/salesforce/donation_validator.rb'