fix: cleanup dist folder before build

Author: 3cp

package.json

@@ -16,7 +16,7 @@
     "version": "standard-changelog && git add CHANGELOG.md",
     "postversion": "git push && git push --tags && npm publish",
     "pretest": "npm run lint",
-    "prepare": "babel src -d dist",
+    "prepare": "del dist && babel src -d dist",
     "nodejs-test": "tape -r @babel/register \"spec/**/*.spec.js\" | tap-dot",
     "coverage": "nyc --reporter=lcov tape -r @babel/register \"spec/**/*.spec.js\" | tap-dot",
     "browser-test": "browserify -t babelify spec/all-browser-spec.js | tape-run | tap-dot",
@@ -57,6 +57,7 @@
     "babel-eslint": "^10.0.1",
     "babelify": "^10.0.0",
     "browserify": "^16.2.3",
+    "del-cli": "^1.1.0",
     "eslint": "^5.9.0",
     "nyc": "^13.1.0",
     "standard-changelog": "^2.0.6",