Upgrade to pybitcointools 1.1.15

### Motivation:
The latest version of `pybitcointools` packs [improved speed]() and a [revamped testsuite](). I just finished a diff between version `1.1` and `1.1.15` without detecting anything that could cause trouble save for one thing: the latest version redefines functions `sum` and `slice`, which shadow python builtins and make `pylint` unhappy. However, there is no usage of these functions in our code; the `pylint` warning is a result of the wildcard imports in `node/ws.py` and `node/orders.py`. Thus, this PR depends on #673.

### Changelog:
* Upgrade to `pybitcointools` version `1.1.15`.