Release wf-common 0.1.15

PRs contained in this release:

https://github.com/WebFilings/wf-common/pull/12


How To +10/QA
-------------

```bash
# skip the following steps if you've already initialized the project
git clone git@github.com:WebFilings/wf-common.git
cd wf-common

# do not skip these!
git fetch && git checkout Release_0.1.15
Run unit tests
```