Fix deasync production build failure

[rozetko]

npm start fails with the following error:

Could not locate the bindings file. Tried:

     → /home/travis/build/hastic/hastic-server/server/node_modules/deasync/build/deasync.node

     → /home/travis/build/hastic/hastic-server/server/node_modules/deasync/build/Debug/deasync.node

     → /home/travis/build/hastic/hastic-server/server/node_modules/deasync/build/Release/deasync.node

     → /home/travis/build/hastic/hastic-server/server/node_modules/deasync/out/Debug/deasync.node

     → /home/travis/build/hastic/hastic-server/server/node_modules/deasync/Debug/deasync.node

     → /home/travis/build/hastic/hastic-server/server/node_modules/deasync/out/Release/deasync.node

     → /home/travis/build/hastic/hastic-server/server/node_modules/deasync/Release/deasync.node

     → /home/travis/build/hastic/hastic-server/server/node_modules/deasync/build/default/deasync.node

     → /home/travis/build/hastic/hastic-server/server/node_modules/deasync/compiled/8.16.1/linux/x64/deasync.node

The error reason is the deasync dependency.

Changes

• resolve bindings to () => require('./deasync.node') in externals
• replaced deasync context with platform-specific path
• add node-loader to the 2nd build-stage

Tested it on node 6 and 8 (both Linux)