Switch to Yarn for dependency management

package.json

@@ -43,14 +43,14 @@
   "scripts": {
     "lint": "jscs . && jshint . --exclude-path .gitignore",
     "standalone": "browserify index.js --standalone Pjax > pjax.js",
-    "build": "npm run standalone && uglifyjs pjax.js -o pjax.min.js",
+    "build": "yarn run standalone && uglifyjs pjax.js -o pjax.min.js",
     "build-debug": "browserify index.js --debug --standalone Pjax > pjax.js",
     "tests": "tape -r ./tests/setup.js \"./tests/**/*.js\"",
-    "test": "npm run lint && npm run tests | tap-spec",
-    "coverage-tests": "npm run tests | tap-nyc",
-    "coverage": "nyc -x \"tests/**\" npm run coverage-tests",
+    "test": "yarn run lint && yarn run tests | tap-spec",
+    "coverage-tests": "yarn run tests | tap-nyc",
+    "coverage": "nyc -x \"tests/**\" yarn run coverage-tests",
     "example": "opn http://localhost:3000/example/ && serve -p 3000 .",
-    "prepublish": "npm run build",
+    "prepublish": "yarn run build",
     "release": "npmpub"
   }
 }