fix: Add missing dev dependency

Author: mrchief

package.json

@@ -74,6 +74,7 @@
     "eslint-plugin-promise": "^3.7.0",
     "eslint-plugin-react": "^7.7.0",
     "eslint-plugin-standard": "^3.0.1",
+    "extract-text-webpack-plugin": "^3.0.2",
     "github-markdown-css": "^2.10.0",
     "husky": "^0.14.3",
     "ignore-styles": "^5.0.1",

yarn.lock

@@ -674,7 +674,7 @@ async@^1.4.0, async@^1.5.0, async@^1.5.2:
   version "1.5.2"
   resolved "https://registry.yarnpkg.com/async/-/async-1.5.2.tgz#ec6a61ae56480c0c3cb241c95618e20892f9672a"
 
-async@^2.1.2, async@^2.6.0:
+async@^2.1.2, async@^2.4.1, async@^2.6.0:
   version "2.6.0"
   resolved "https://registry.yarnpkg.com/async/-/async-2.6.0.tgz#61a29abb6fcc026fea77e56d1c6ec53a795951f4"
   dependencies:
@@ -4083,6 +4083,15 @@ extglob@^2.0.4:
     snapdragon "^0.8.1"
     to-regex "^3.0.1"
 
+extract-text-webpack-plugin@^3.0.2:
+  version "3.0.2"
+  resolved "https://registry.yarnpkg.com/extract-text-webpack-plugin/-/extract-text-webpack-plugin-3.0.2.tgz#5f043eaa02f9750a9258b78c0a6e0dc1408fb2f7"
+  dependencies:
+    async "^2.4.1"
+    loader-utils "^1.1.0"
+    schema-utils "^0.3.0"
+    webpack-sources "^1.0.1"
+
 [email protected]:
   version "1.3.0"
   resolved "https://registry.yarnpkg.com/extsprintf/-/extsprintf-1.3.0.tgz#96918440e3041a7a414f8c52e3c574eb3c3e1e05"