test: remove microtime, use node 4+ only

Author: epoberezkin

.travis.yml

@@ -1,9 +1,8 @@
 language: node_js
 node_js:
-  - "0.10"
-  - "0.12"
   - "4"
   - "6"
+  - "7"
   - "8"
 after_script:
   - coveralls < coverage/lcov.info

package.json

@@ -31,7 +31,6 @@
     "deep-equal": "^1.0.1",
     "eslint": "^4.0.0",
     "lodash": "^4.17.4",
-    "microtime": "^2.1.5",
     "mocha": "^3.4.2",
     "nano-equal": "^1.0.1",
     "nyc": "^11.0.2",