ability to take main file from package json if bower.json is not exists

[mogadanez]

Some components author ( cferdinandi/smooth-scroll#155 ), does not want to
add official support specific package managers.

In this case, it not have bower.json file with specified main property. So, after bowerRequirejs task execution it takes first file in package root folder - in this case it is gulp.js

However, in this particular module, main file is specified in package json, and it will be nice if bowerRequirejs task can fallback to it when resolve mappings

[sindresorhus]

That logic should really be in Bower itself, but sure, a good PR (to the bower-requirejs module, not here) with tests would be accepted.