Fix build in node 10

Building the project and running tests in node 10 led to the
following error:

```
$ npm test

> @ test /Users/rzueger/Development/odch/flightbox
> gulp test

gulp[18899]: ../src/node_contextify.cc:631:static void node::contextify::ContextifyScript::New(const FunctionCallbackInfo<v8::Value> &): Assertion `args[1]->IsString()' failed.
 1: node::Abort() [/usr/local/bin/node]
 2: node::Assert(char const* const (*) [4]) [/usr/local/bin/node]
 3: node::contextify::ContextifyScript::New(v8::FunctionCallbackInfo<v8::Value> const&) [/usr/local/bin/node]
 4: v8::internal::FunctionCallbackArguments::Call(v8::internal::CallHandlerInfo*) [/usr/local/bin/node]
 5: v8::internal::MaybeHandle<v8::internal::Object> v8::internal::(anonymous namespace)::HandleApiCallHelper<true>(v8::internal::Isolate*, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::FunctionTemplateInfo>, v8::internal::Handle<v8::internal::Object>, v8::internal::BuiltinArguments) [/usr/local/bin/node]
 6: v8::internal::Builtin_Impl_HandleApiCall(v8::internal::BuiltinArguments, v8::internal::Isolate*) [/usr/local/bin/node]
 7: 0x1d7692b8427d
 8: 0x1d7692b8f755
 9: 0x1d7692c0bc20
10: 0x1d7692b944f7
11: 0x1d7692b944f7
12: 0x1d7692b944f7
```

Fixed by removing package-lock.json and generating a new one (see
gulpjs/gulp#2162 (comment)).