chore: update prettier dependency version

1. Only 1.17+ supports yarn plug'n'play
2. We pinned the prettier version because there were breaking changes in
minor version before. Now that we'd catch errors from the prettier call,
it's no longer a concern.

Author: haoqunjiang

Diff for: package.json

@@ -61,7 +61,7 @@
     "merge-source-map": "^1.1.0",
     "postcss": "^7.0.14",
     "postcss-selector-parser": "^5.0.0",
-    "prettier": "1.16.3",
+    "prettier": "^1.18.2",
     "source-map": "~0.6.1",
     "vue-template-es2015-compiler": "^1.9.0"
   }

Diff for: yarn.lock

@@ -4029,10 +4029,10 @@ prelude-ls@~1.1.2:
   resolved "https://registry.yarnpkg.com/prelude-ls/-/prelude-ls-1.1.2.tgz#21932a549f5e52ffd9a827f570e04be62a97da54"
   integrity sha1-IZMqVJ9eUv/ZqCf1cOBL5iqX2lQ=
 
-prettier@1.16.3:
-  version "1.16.3"
-  resolved "https://registry.yarnpkg.com/prettier/-/prettier-1.16.3.tgz#8c62168453badef702f34b45b6ee899574a6a65d"
-  integrity sha512-kn/GU6SMRYPxUakNXhpP0EedT/KmaPzr0H5lIsDogrykbaxOpOfAFfk5XA7DZrJyMAv1wlMV3CPcZruGXVVUZw==
+prettier@^1.18.2:
+  version "1.18.2"
+  resolved "https://registry.yarnpkg.com/prettier/-/prettier-1.18.2.tgz#6823e7c5900017b4bd3acf46fe9ac4b4d7bda9ea"
+  integrity sha512-OeHeMc0JhFE9idD4ZdtNibzY0+TPHSpSSb9h8FqtP+YnoZZ1sl8Vc9b1sasjfymH3SonAF4QcA2+mzHPhMvIiw==
 
 pretty-format@^24.8.0:
   version "24.8.0"