fix: explicitly depend on mocha

        - I'm seeing it automatically brought in on win32, but it's
        missing on Linux

Author: fluffynuts

package.json

@@ -29,6 +29,7 @@
     "chai": "^3.4.0",
     "coveralls": "^2.11.4",
     "istanbul": "^0.4.0",
+    "mocha": "^3.2.0",
     "mocha-runner": "^1.1.1",
     "npm-run-all": "^4.0.2",
     "rewire": "^2.5.1",