fixed getMochaOpts() for no-arg options at the end of argv

I noticed that the `--bail` option was ignored when I tried this:

    promises-aplus-tests my_adapter.js --bail

From a quick glance at the code, it's clear this is because `bail.split(' ')[1]` is `undefined`, not an empty string. The fix to `getMochaOpts` is simple: check the 'value' of the option for falsiness rather than equality with `""`.

Now, some folks don't like PRs without tests. So I have two proposals: one is this PR, which is just the simple one-line fix. The other proposal is a separate PR, in which I also add some tests for the `getMochaOpts` method.

The downside to the PR with tests, of course, is that it involves pulling the method out of cli.js to make it testable and sort of disrupts (in my opinion) what is otherwise a very clean, simple project structure.

Personally I don't have a preference between the two; but I wanted to offer both so you can choose between them.