bump middleman dep

[dschobel]

One of middleman's transitive deps (sprockets 2.12.4) has a known security vulnerability so we bump middleman to a version which doesn't have the issue.

Known vulnerability found
CVE-2018-3760
High severity
Specially crafted requests can be used to access files that exist on the filesystem that is outside an application's ...

Gemfile.lock update suggested:
sprockets ~> 2.12.5
Always verify the validity and compatibility of suggestions with your codebase.

[bryce-anderson]

Presuming it still builds.

[cacoco]

Can you try to run the blog locally with these changes? Just to make sure.

[dschobel]

I tested it locally and it rendered fine. It failed in travisci because we need to bump the ruby version: https://travis-ci.org/finagle/finagle.github.io/jobs/402802163

[dschobel]

we're good: https://travis-ci.org/finagle/finagle.github.io/builds/402819412