4.2.7 breaks programmatic interface

[chohner]

After updating to v4.2.7, we are seeing the following error when using the API (our implementation):

/node_modules/slide/lib/async-map.js:47
    if (--a === 0) cb_.apply(null, [errState].concat(data))
                       ^
TypeError: this.writeOutput is not a function
    at /node_modules/license-checker-rseidelsohn/lib/index.js:732:18
    at /node_modules/read-installed-packages/read-installed.js:139:5
    at /node_modules/read-installed-packages/read-installed.js:267:14
    at cb (/node_modules/slide/lib/async-map.js:47:24)
    at /node_modules/read-installed-packages/read-installed.js:267:14
    at cb (/node_modules/slide/lib/async-map.js:47:24)
    at /node_modules/read-installed-packages/read-installed.js:245:16
    at /node_modules/slide/lib/async-map.js:52:35
    at Array.forEach (<anonymous>)
    at /node_modules/slide/lib/async-map.js:52:11

Seems to be connected to #81, where for the first time a methods of this is called, which in our environment references a global this (not the exports object).

Suggestion: Changing it to exports.writeOutput seems to work for us. (I made a corresponding PR: #96)