v2016.04.18.01

• Thanks to @Dominator008, vm_args was added to java_test documentation: https://buckbuild.com/rule/java_test.html
• xctests are now streamed to Buck instead of waiting until completion
• A flatten_dicts method was added for use in build files: https://buckbuild.com/function/flatten_dicts.html
• Buck can now fail the build when using header files that are untracked by build files: https://buckbuild.com/concept/buckconfig.html#cxx.untracked_headers
• Buck now has very basic support for the AppleTV platform
• Buck now caches its Action Graph, which is part of the Parsing phase. Large projects should see this step speed up.
• Cross-cell references can now be called with @ or +. The latter is useful for command-line invocations.
• The repl command was added: https://buckbuild.com/command/repl.html
• Support for Android NDK r11 and newer was added.
• Buck now allows overriding of the command to package for apple_binary rules: https://buckbuild.com/concept/buckconfig.html#apple.*_package_command