MAGETWO-69434: Update composer.json in functional tests to support latest version

 - Updated version of MTF

Author: YevSent

dev/tests/functional/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "magento/mtf": "1.0.0-rc54",
+        "magento/mtf": "1.0.0-rc55",
         "php": "7.0.2|~7.0.6|~7.1.0",
         "phpunit/phpunit": "~4.8.0|~5.5.0",
         "phpunit/phpunit-selenium": ">=1.2"