buildifier path doesn't work after upgrading bazel

[alexeagle]

observed in #59

[alexeagle]

also on linux:
dist/bin/external/com_github_bazelbuild_buildtools/buildifier/linux_amd64_stripped/buildifier

I think that Bazel 0.9 changed the output directory layout to include the architecture, and now the binary path for a target is no longer predictable :(

[alexeagle]

Have a workaround in #58

[alexeagle]

Now that buildifier publishes pre-built binaries, we don't care about paths to find it anymore