Fix uninitialized constant error

[thorin]

Added require_relative 'flint_action' to fix an unitialized constant error:

[✔] 🚀
[17:14:16]: Error loading plugin 'fastlane-plugin-flint': uninitialized constant Fastlane::Actions::FlintAction
Did you mean?  Fastlane::Actions::PilotAction                                                                                                                                                                                                       
+-------------------------+---------+------------------+
|                     Used plugins                     |
+-------------------------+---------+------------------+
| Plugin                  | Version | Action           |
+-------------------------+---------+------------------+
| fastlane-plugin-cordova | 2.2.1   | cordova          |
| fastlane-plugin-flint   | 0.1.0   | No actions found |
+-------------------------+---------+------------------+