Use mixin 0.8.1. Should fix problems with mixin not working properly with latest modlauncher.

Signed-off-by: cpw <cpw+github@weeksfamily.ca>

build.gradle

@@ -451,7 +451,7 @@ project(':forge') {
         installer 'org.apache.logging.log4j:log4j-core:2.11.2'
         installer 'net.minecrell:terminalconsoleappender:1.2.+'
         installer 'net.sf.jopt-simple:jopt-simple:5.0.4'
-        installer 'org.spongepowered:mixin:0.8'
+        installer 'org.spongepowered:mixin:0.8.1'
         fmllauncherImplementation 'com.google.guava:guava:21.0'
         fmllauncherImplementation 'com.google.code.gson:gson:2.8.0'
         fmllauncherImplementation 'org.lwjgl:lwjgl:3.2.2'