Update API compatibility testing documentation

The current documentation says to use this snippet to run API compatibility tests:

const apiCompatibilityChecks = require('opentracing/test/api_compatibility.js');
apiCompatibilityCheck(() => new CustomTracer());

However, since version 0.14, it should be instead:

const apiCompatibilityChecks = require('opentracing/lib/test/api_compatibility.js').default;
apiCompatibilityCheck(() => new CustomTracer());

See: https://github.com/opentracing/opentracing-javascript#api-compatibility-testing

[rochdev]

I don’t see any problems with requiring .default as long as it’s documented.

[pavolloffay]

closing per #85