Change istanbul call with call to cli.js

See: gotwarlost/istanbul#44
Now krew can be used with node 0.12 and later

package.json

@@ -13,7 +13,7 @@
     "url": "https://github.com/KurtPattyn/krew.git"
   },
   "engines": [
-    "node >=0.11.0"
+    "node >=0.12.0"
   ],
   "dependencies": {
     "bluebird": "^2.10.1",
@@ -29,11 +29,11 @@
     "mocha": "^2.3.0"
   },
   "scripts": {
-    "test": "node_modules/mocha/bin/mocha --bail --reporter spec --check-leaks test/",
-    "test-cov": "istanbul cover node_modules/mocha/bin/_mocha -- --reporter spec --check-leaks test/",
-    "test-ci": "istanbul cover node_modules/mocha/bin/_mocha --report lcovonly -- --reporter spec --check-leaks test/",
+    "test": "node_modules/mocha/bin/mocha --harmony --require test/support/env --bail --reporter spec --check-leaks test/",
+    "test-ci": "node --harmony node_modules/istanbul/lib/cli.js cover node_modules/mocha/bin/_mocha --report lcovonly -- --harmony --require test/support/env --reporter spec --check-leaks test/",
+    "test-cov": "node --harmony node_modules/istanbul/lib/cli.js cover node_modules/mocha/bin/_mocha -- --require test/support/env --reporter spec --check-leaks test/",
     "check-coverage": "istanbul check-coverage --statements 100 --lines 100 --functions 100 --branches 100",
-    "benchmark": "node benchmarks/benchmark",
+    "benchmark": "node --harmony benchmarks/benchmark",
     "make-docs": "./node_modules/.bin/jsdoc -c jsdoc.conf",
     "check-style": "./node_modules/jscs/bin/jscs .",
     "code-analysis": "./node_modules/jshint/bin/jshint ."