fix(runner): prevent source-map-support module from exiting process on `...

This is a quick patch for issue #876 which allows us to let Cucumber keep going even when a test step fails (the problem seems to mostly happen when it's a protractor promise that fails).

However, I can still see that the error stacks showing CoffeeScript source files are displayed with line numbers that actually match the js generated code, not the CoffeeScript code.
So I think that the source-map-support configuration is not correct when used with Cucumber js, but this should be fixed in another PR I reckon.

Done.