Conditionally purge on deploys

We can configure this within Travis

bin/configure-fastly.js

@@ -177,10 +177,12 @@ async.auto({
             fastly.activateVersion(results.version, function (err, response) {
                 if (err) throw new Error(err);
                 process.stdout.write('Successfully configured and activated version ' + response.number + '\n');
+                if (process.env.FASTLY_PURGE_ALL) {
                     fastly.purgeAll(FASTLY_SERVICE_ID, function (err) {
                         if (err) throw new Error(err);
                         process.stdout.write('Purged all.\n');
                     });
+                }
             });
         }
     }