Dedup dependencies

[emschwartz]

There are a lot of different versions of bignumber.js and oer-utils floating around our modules. It would be good to clean those up and get them all using the same versions so we don't need to install all of them.

ilp-protocol-stream git:es-docs ❯ npm ls --production
ilp-protocol-stream@0.1.2 .../ilp-protocol-stream
├── @types/node@9.6.1
├── bignumber.js@6.0.0
├─┬ debug@3.1.0
│ └── ms@2.0.0
├─┬ ilp-packet@2.2.0
│ ├── bignumber.js@5.0.0
│ ├── extensible-error@1.0.2
│ ├── long@3.2.0
│ └── oer-utils@1.3.4
├─┬ ilp-plugin@3.1.0
│ └─┬ ilp-plugin-btp@1.1.8
│   ├── base64url@2.0.0
│   ├─┬ btp-packet@1.2.0
│   │ ├── base64url@2.0.0 deduped
│   │ ├── bignumber.js@4.1.0
│   │ ├── dateformat@2.2.0
│   │ ├── oer-utils@1.3.4
│   │ └── uuid-parse@1.0.0
│   ├── debug@3.1.0 deduped
│   ├── eventemitter2@5.0.1
│   └─┬ ws@3.3.3
│     ├── async-limiter@1.0.0
│     ├── safe-buffer@5.1.1
│     └── ultron@1.1.1
├─┬ ilp-protocol-ildcp@1.0.0
│ ├── ilp-packet@2.2.0 deduped
│ └── oer-utils@1.3.4
├─┬ oer-utils@2.0.1
│ └── bignumber.js@6.0.0 deduped
└─┬ source-map-support@0.5.4
  └── source-map@0.6.1