[BugFix] Migration log index size fix

[rodriguez-loic]

migrations/src/Migrations.php

Line 734 in 69c1ea9

'size' => 255,

This line should either be configurable dynamically or modified back to a value <=191. (maybe a check on the collation settings from the configuration could do it)
Because of the evolution of MySQL Engines, the default utf8 charset used today is utf8mb4 which doesn't allow such length for VARCHAR type.

This issue was already fixed in an old version of phalcon devtools: phalcon/phalcon-devtools#1293