remove build from .gitignore as we need BUILD files
remove build from .gitignore as we need BUILD files

For more about BUILD files, see:
https://www.pantsbuild.org/docs/targets#build-files

Benjy Weinberger, one of the pants maintainers, pointed out why git
was ignoring the BUILD files.

Plexxi/st2,Plexxi/st2,nzlosh/st2,StackStorm/st2,nzlosh/st2,StackStorm/st2,nzlosh/st2,Plexxi/st2,Plexxi/st2,StackStorm/st2,nzlosh/st2,StackStorm/st2