Update sprockets in Gemfile.lock (#48)
Update sprockets in Gemfile.lock (#48)

GitHub alarms me that buoys includes sprockets 3.1.7 and it has known vulnerability,
[CVE-2018-3760](https://groups.google.com/forum/#!topic/ruby-security-ann/2S9Pwz2i16k) and I
suppress the alarm.

```
We found a potential security vulnerability in one of your dependencies.
A dependency defined in Gemfile.lock has known security vulnerabilities and should be updated.
```
muryoimpl/buoys,muryoimpl/buoys,muryoimpl/buoys