Remove requirement that a dpendency is present

 #24 references a potential use-case.

I have a project where I'm trying to use only PARTS of a bower install in a sub folder. 

    |--
       | -- bower_components (original)
            |-- bower_package_1
            |-- bower_package_2
            |-- bower_package_3
       | -- sub_project
            |-- bower_components (copied)
                |-- bower_package_1
                |-- bower_package_3
            |-- index.html
       |-- bower.json
       |-- Gruntfile.js

In the Gruntfile, I do this:

    sub_project: {
        directory: 'sub_project/bower_components',
        src: [
          'sub_project/*.html',
          'stylesheets/*.scss'
        ],
        // Optional:
        // ---------
        cwd: 'sub_project',

The Gruntfile runs the bower install and then copies the bower packages around using a copy job. I'm hoping to see sub_project/index.html reflect only the parts that are copied in. However, right now your tool will bomb out due to the error:

    `Cannot read property 'main' of undefined`

This happens on line 146 in lib/detect-dependencies.js (see change). The fix is to simply omit this block when the config file isn't found. 

It is a one line change, but may have circumvent purposeful designs. An alternative is to include an or block in that if condition:

    if(componentConfigFile || config.ensureDependencyInstalled) { 

And set that config value to true by default.