Run buckd by default if $USE_BUCKD is defined.

Test Plan:
buck build buck
yolo4
Ensure changes to resources cause java_library targets to be repacked.

Summary:
Buck should repackage java_library targets if their resources change, even if that resource
has an ABI that itself hasn't changed.

Test Plan: buck test --all
Expose hashes of .class files that end up in a DexWithClasses.

Summary:
PreDexedFilesSorter uses this to create a map of dex input paths to their hash,
just like HashInputJarsToDexStep. Everything will slowly come together perfectly :)

Test Plan:
build messenger.
Add a call to `getDexInputsHashes()` before returning PreDexedFilesSorter.Result,
attach debugger and verify hashes are getting calculated.
SmartDexingStep no longer relies on the crc of zip entries to hash input jars.

Summary:
The hashes of inputs to be dexed are passed from above into SmartDexingStep,
which it uses to decide whether a dex needs to be recreated. Since earlier, it
was using the zip entry crc for this, it also used to check for timestamps on
the hash output files (arghhh) which basically meant smart dexing never worked
on the first run post pulling-everything-from-cache. Now everything is awesome!

Test Plan:
buck build fb4a, messenger
change a java file, build again, verify only one of the dex files is recreated.
unit tests.
macosx_binary copy frameworks into .app/Frameworks

Summary: These frameworks were actually being copied into the Products/ folder, not Products/App.app/Content/Frameworks/

Test Plan: Update unit test, pass. Build Diamond editor, move to temp folder, execute. It doesn't crash now.
Update build-tools version for Travis

Summary:
We hit an issue in Travis where the version of dx from built-tools-v18 doesn't
like our java7 class files.  https://travis-ci.org/facebook/buck/builds/23236332
Update to build-tools-v19.0.3 to fix it.

Test Plan:
Ran the travis test script locally and saw these failures go away. There was
one other seemingly unrelated failure, but it could be a flaky test, not sure
yet.