Upgrade to pyelliptic 1.5.4.

### Motivation:
The [pyelliptic](https://pypi.python.org/pypi/pyelliptic) package has been upgraded in PyPI, shipping a [cleaner codebase](https://github.com/yann2192/pyelliptic/blob/master/CHANGES.txt) (thanks to @dionyziz) and some import fixes. We can safely upgrade.

### Changelog:
* Upgrade requirements to `pyelliptic` version `1.5.4`.