We recently changed the way that we check whether babel, lessc, rollup

and uglifyjs binaries exist in the node modules path when building

Pipeline settings. We used to hard-code paths like

<node_modules_path>/.bin/rollup and check for their existence, but now

we use the npm exec command.
However, we invoke the command from the current working directory, when we

should actually be invoking it from the parent directory of the node

modules path.
This was made evident when trying to run Power Pack's unit tests. Power Pack

relies on Review Board's node modules, but since the npm exec command was

ran from the Power Pack repository, it was looking in Power Pack's node

modules instead of Review Board's.
This change fixes this so that the binaries are actually checked for in

the node modules paths, instead of the current working directory. If a

binary isn't found in any of the configured node module paths then we

raise the error.

Ran unit tests.
Was able to run Power Pack unit tests without hitting the

ImproperlyConfigured error for rollup that I was hitting before.