Update source-map dependency

Firstly thanks very much for this excellent library.

I am running a Nodejs app and am hitting the following error when using this library:
```
You must provide the URL of lib/mappings.wasm by calling SourceMapConsumer.initialize({ 'lib/mappings.wasm': ... }) before using SourceMapConsumer
```

* A dependency of yours "source-map" detects the runtime (Nodejs vs browser) in a broken way. That library then throws an exception if some client-side-specific code is not run.
* The "check" checks for the presence of a variable "fetch" and the reasonable thinking is that only browsers will have fetch. The issue is that in my case and in some others (see [here](mozilla/source-map#349)) 3rd party libraries can define a variable "fetch" which triggers the described error.

None

package.json

@@ -11,7 +11,7 @@
  },
  "dependencies": {
  "bluebird": "^3.5.2",
- "source-map": "^0.7.3"
+ "source-map": "^0.8.0-beta.0"
  },
  "description": "V8 stack traces with optional source map resolution.",
  "devDependencies": {