Build with Java 8

Change-Id: Ia736fb268e815a7aea66a2b0d96ff6f5ba8ae211

.buckconfig

diff --git a/.buckconfig b/.buckconfig
index 4e88473..f163b1c 100644
--- a/.buckconfig
+++ b/.buckconfig
@@ -3,6 +3,10 @@
   install = //:install
   deploy = //:deploy
 
+[java]
+  source_level = 8
+  target_level = 8
+
 [buildfile]
   includes = //bootstrap.defs \
       //bucklets/java_sources.bucklet \