old devkit-core causes error on build

[yofreke]

seems like probably an incompatibility with an older devkit-core. Reported by @fairfieldt

  [CompanionMonitorServer]  UIClient Connected
        build failed
        [error]  exception: TypeError: Object.keys called on non-object
        at Function.keys (native)
        at /home/weeby/dev/devkit/src/build/index.js:61:29
        at Array.reduce (native)
        at SuperGroup.<anonymous> (/home/weeby/dev/devkit/src/build/index.js:60:23)
        at SuperGroup._execNextStep (/home/weeby/dev/devkit/node_modules/ff/lib/ff.js:365:10)
        at Group.done (/home/weeby/dev/devkit/node_modules/ff/lib/ff.js:69:17)
        at /home/weeby/dev/devkit/node_modules/ff/lib/ff.js:136:34
        at tryCatcher (/home/weeby/dev/devkit/node_modules/bluebird/js/main/util.js:26:23)
        at Promise.successAdapter (/home/weeby/dev/devkit/node_modules/bluebird/js/main/nodeify.js:23:30)
        at Promise._settlePromiseAt (/home/weeby/dev/devkit/node_modules/bluebird/js/main/promise.js:579:21)
        at Promise._settlePromises (/home/weeby/dev/devkit/node_modules/bluebird/js/main/promise.js:697:14)
        at Async._drainQueue (/home/weeby/dev/devkit/node_modules/bluebird/js/main/async.js:123:16)
        at Async._drainQueues (/home/weeby/dev/devkit/node_modules/bluebird/js/main/async.js:133:10)
        at Immediate.Async.drainQueues [as _onImmediate] (/home/weeby/dev/devkit/node_modules/bluebird/js/main/async.js:15:14)
        at processImmediate [as _immediateCallback] (timers.js:367:17)
        [error]  error while building TypeError: Object.keys called on non-object
        at Function.keys (native)
        at /home/weeby/dev/devkit/src/build/index.js:61:29
        at Array.reduce (native)
        at SuperGroup.<anonymous> (/home/weeby/dev/devkit/src/build/index.js:60:23)
        at SuperGroup._execNextStep (/home/weeby/dev/devkit/node_modules/ff/lib/ff.js:365:10)
        at Group.done (/home/weeby/dev/devkit/node_modules/ff/lib/ff.js:69:17)
        at /home/weeby/dev/devkit/node_modules/ff/lib/ff.js:136:34
        at tryCatcher (/home/weeby/dev/devkit/node_modules/bluebird/js/main/util.js:26:23)
        at Promise.successAdapter (/home/weeby/dev/devkit/node_modules/bluebird/js/main/nodeify.js:23:30)
        at Promise._settlePromiseAt (/home/weeby/dev/devkit/node_modules/bluebird/js/main/promise.js:579:21)
        at Promise._settlePromises (/home/weeby/dev/devkit/node_modules/bluebird/js/main/promise.js:697:14)
        at Async._drainQueue (/home/weeby/dev/devkit/node_modules/bluebird/js/main/async.js:123:16)
        at Async._drainQueues (/home/weeby/dev/devkit/node_modules/bluebird/js/main/async.js:133:10)
        at Immediate.Async.drainQueues [as _onImmediate] (/home/weeby/dev/devkit/node_modules/bluebird/js/main/async.js:15:14)
        at processImmediate [as _immediateCallback] (timers.js:367:17)
  Unhandled rejection TypeError: undefined is not a function