Fix compatibility issues with GTCE 1.15

[Exaxxion]

Fixes #14

• Switches to GTCE's fork of ForgeGradle 2.3 to have a working build.
• Updates GTCE dependency to 1.15.0.721, and mandates at least that version at runtime to avoid problems.
• Corrects references to removed methods that cause #14 .
• Does not replace any deprecated methods that are internally remapped to the new implementation, but this should be done to avoid more problems in the future.

Thanks also to @ALongStringOfNumbers for working with me on this.

[eutro]

Looks good, thank you very much.