fix card expiration date format for Pagar.me Gateway

[ccarruitero]

fix #2534

$ rake test TEST=test/unit/gateways/pagarme_test.rb 
Started
................
Finished in 0.028124156 seconds.
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16 tests, 119 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
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568.91 tests/s, 4231.24 assertions/s

$ rake test TEST=test/remote/gateways/remote_pagarme_test.rb 
Started
......F
=======================================================================================================================================================================================
     148: 
     149:     response = gateway.purchase(@amount, @credit_card, @options)
     150:     assert_failure response
  => 151:     assert_match %r{401 Authorization Required}, response.message
     152:   end
     153: 
     154:   def test_transcript_scrubbing
/home/pavle/tmp/active_merchant/test/remote/gateways/remote_pagarme_test.rb:151:in `test_invalid_login'
Failure: test_invalid_login(RemotePagarmeTest):
  </401 Authorization Required/> was expected to be =~
  <"Invalid environment">.
=======================================================================================================================================================================================
........
Finished in 16.44644862 seconds.
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15 tests, 46 assertions, 1 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
93.3333% passed
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0.91 tests/s, 2.80 assertions/s

Failing in remote test is unrelated to the change.

[github-actions]

To provide a cleaner slate for the maintenance of the library, this PR/Issue is being labeled stale after 60 days without activity. It will be closed in 14 days unless you comment with an update regarding its applicability to the current build. Thank you!