Remove jcenter as a default maven repo. This will make builds faster and more reliable. It is trivial to add back manually for projects that require it.

Closes #314

src/main/java/net/fabricmc/loom/configuration/CompileConfiguration.java

@@ -159,7 +159,6 @@ public final class CompileConfiguration {
 			});
 
 			project1.getRepositories().mavenCentral();
-			project1.getRepositories().jcenter();
 
 			LoomDependencyManager dependencyManager = new LoomDependencyManager();
 			extension.setDependencyManager(dependencyManager);