Remove dependency on microtime

Per balderdashy#3211, this version of microtime
doesn't work with Node 4, and is trivially replaceable with `process.hrtime`
from the standard library.

Patch mostly applied from commit b0b94c9 in
the upstream repository. You probably want to view this diff with whitespace
changes disabled, the actual diff is very short.

package.json

@@ -69,7 +69,6 @@
     "coffee-script": "1.7.1",
     "expect.js": "0.2.0",
     "istanbul": "*",
-    "microtime": "0.6.0",
     "mocha": "2.3.4",
     "npm": "2",
     "portfinder": "0.2.1",