Dogfood bintray for pants support binaries.

Testing Done:
Forced a local re-bootstrap of thrift and protoc:
```console
$ rm -rf ~/.pants.d
$ pants.dev goal clean-all gen testprojects/::
*** Running pants in dev mode from /home/jsirois/dev/3rdparty/jsirois-pants3/src/python/pants/bin/pants_exe.py ***

15:30:59 00:00 [main]
               (To run a reporting server: ./pants goal server)
15:30:59 00:00   [bootstrap]
15:31:00 00:01   [setup]
15:31:00 00:01     [parse] INFO] Attempting to fetch protoc binary from: https://dl.bintray.com/pantsbuild/bin/build-support/bin/protobuf/linux/x86_64/2.4.1/protoc ...
 INFO] Fetched protoc binary from: https://dl.bintray.com/pantsbuild/bin/build-support/bin/protobuf/linux/x86_64/2.4.1/protoc .
 INFO] Attempting to fetch thrift binary from: https://dl.bintray.com/pantsbuild/bin/build-support/bin/thrift/linux/x86_64/0.5.0-finagle/thrift ...
 INFO] Fetched thrift binary from: https://dl.bintray.com/pantsbuild/bin/build-support/bin/thrift/linux/x86_64/0.5.0-finagle/thrift .

               Executing tasks in goals: clean-all -> bootstrap -> imports -> gen
...
$ ~/.pants.d/bin/protobuf/linux/x86_64/2.4.1/protoc --version
libprotoc 2.4.1
$ ~/.pants.d/bin/thrift/linux/x86_64/0.5.0-finagle/thrift -version
Thrift version 0.5.0
```

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/38865985

Bugs closed: 715

Reviewed at https://rbcommons.com/s/twitter/r/1208/