Update buildscript & fix bugs

[NeRdTheNed]

Sorry for the large amount of changes, I got a bit carried away with scope creep!

This PR:

• Updates the Gradle buildscript to use ForgeGradle 5.1.x, as well as updating the dependencies and build logic.
• Fixes #7 by registering the EntitySimpleArrow class.
• Fixes EntitySimpleArrows not having the shootingEntity set client-side, which caused issues after they were registered.
• Creates a helper Enum (EnumArrowLogics) to store the used IArrowLogics, to facilitate saving the set IArrowLogic for shot EntitySimpleArrows.
• Fixes EntitySimpleArrows not saving their set IArrowLogic.
• Fixes a bug which caused EntitySimpleArrows to emit potion particles when saving and reloading. I don't know why this bug occurred.

Known issues:

• When a EntitySimpleArrow is saved and reloaded, it does not remember the shootingEntity. This is the same as vanilla behaviour, with the most noticeable being that when an ender arrow is shot and the game is saved and reloaded, the ender arrow will not teleport the player upon landing. Working around this would possibly require keeping track of players that login, and checking the UUID using a EntityDataManager to set the shootingEntity for EntitySimpleArrows which have a saved UUID which matches a player that has logged in. I decided this change would be probably too large for this PR, and that the bug was ultimately very minor. I can open an issue for this bug if this PR gets merged.
• ForgeGradle 5.1.x doesn't seem to be applying the access transformers of mod dependencies when running in development environments. This means that when runClient is ran, the game crashes before it can load to the main menu, as JEI tries to use something it assumes has been transformed. I might make a bug report to the ForgeGradle repository if this isn't a known issue for them.
• You'll likely have to update some bits of the buildscript to account for build server changes. I haven't tested if the CurseGradle logic still works.

[NeRdTheNed]

Hey @Darkhax, just wondering if you have seen this PR, as I know from experience that GitHub often fails to notify people when PRs are opened.

[Darkhax]

Thanks for the PR and the reminder, I got the initial email but it got buried. I appreciate the work and will look into it in a few minutes.

[Darkhax]

The access transformer issue is known but it looks like they're not interested in supporting these older versions. There is a project called FancyGradle that offers some patches to fix these issues though. I will look into updating the build script with this and a few other fixes.