Update to testify 0.5.5

  * Fix failure reporting with only one slave in client/server model
    (asottile, #240, #264, #274)
  * Dynamically annotate unittests using catbox data (astephen2, #241)
  * Fix regressions for windows (jareyes409, #245)
  * Enable retries in sql reporter for SQL connection (bukzor, #249)
  * Fix (run, test) ordering check in test_runner_server_test (asottile, #255)
  * Fix flakey tests that depended on order (asotille, #257)
  * Fix timing out test (asottile, #262)
  * Standardize retry options (bukzor, #266)
  * Fix flakey test with sql reporter (bukzor, #267)
  * Use flake8 (astephen2, #271)