Bump mocha from 3.0.2 to 3.1.0

Bumps [mocha](https://github.com/freerange/mocha) from 3.0.2 to 3.1.0.
- [Changelog](https://github.com/freerange/mocha/blob/main/RELEASE.md)
- [Commits](freerange/mocha@v3.0.2...v3.1.0)

---
updated-dependencies:
- dependency-name: mocha
  dependency-version: 3.1.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -158,7 +158,7 @@ GEM
     minitest (6.0.2)
       drb (~> 2.0)
       prism (~> 1.5)
-    mocha (3.0.2)
+    mocha (3.1.0)
       ruby2_keywords (>= 0.0.5)
     nenv (0.3.0)
     net-imap (0.6.2)