feat: Introduce disableFetchRevisions config flag

If you pass this flag then `fetchRevisions` doesn't attempt to fetch _any_ revisions
from S3. This is useful because fetching the revisions from S3 can take a long time
(see #120) and if you don't have any reason to use them then this time is wasted.

In our use case I have hooked this flag up to an environment variable. e.g. - in `deploy.js`:

```
disableFetchRevisions: process.env.DISABLE_FETCH_REVISIONS
```

Then we can set that environment variable when we are calling the `activate`
command and leave it off at any other time (so e.g. `ember deploy:list` works as
expected - if you have the time to wait)

index.js

@@ -107,6 +107,13 @@ module.exports = {
       },
 
       fetchRevisions: function(context) {
+        if (this.readConfig('disableFetchRevisions')) {
+          this.log('fetchRevisions - not fetching any revisions', { verbose: true });
+          return {
+            activations: []
+          };
+        };
+
         return this._list(context)
           .then(function(revisions) {
             return {