Compiling source with java 8

Whenever I try to do

gradlew build

I get the following error:

warning: [options] bootstrap class path not set in conjunction with -source 1.6

C:\Users\----\Documents\CreativeWorkspace\MHFC\build\sources\java\mhfc\net\client\model\mhfcmodel\ModelMHFC.java:130:

error: try-with-resources is not supported in -source 1.6

                try (InputStream is = Minecraft.getMinecraft().getResourceManager()

                    ^

  (use -source 7 or higher to enable try-with-resources)

Now I know that it says java 7 there but it will hit a java 8 section of the code eventually. How can I fix this issue?

PS: My JAVA_HOME path is set to a java8u5 jdk, so definitly no problem there.

PPS: No, I can't switch to version 1.6