fix: upgrade postcss from 5.2.15 to 5.2.18

Snyk has created this PR to upgrade postcss from 5.2.15 to 5.2.18.

See this package in yarn:
postcss

See this project in Snyk:
https://app.snyk.io/org/slack-sidekick/project/dbf711db-aec5-4f16-8fba-4df5f0bb74b0?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot

package.json

@@ -31,7 +31,7 @@
     "cssesc": "^3.0.0",
     "loader-utils": "^1.0.2",
     "normalize-path": "^3.0.0",
-    "postcss": "^5.2.15",
+    "postcss": "^5.2.18",
     "postcss-modules-local-by-default": "^1.1.1",
     "postcss-modules-scope": "^1.0.2"
   }

yarn.lock

@@ -100,7 +100,7 @@ postcss-modules-scope@^1.0.2:
     css-selector-tokenizer "^0.6.0"
     postcss "^5.0.4"
 
-postcss@^5.0.4, postcss@^5.2.15:
+postcss@^5.0.4:
   version "5.2.15"
   resolved "https://registry.yarnpkg.com/postcss/-/postcss-5.2.15.tgz#a9e8685e50e06cc5b3fdea5297273246c26f5b30"
   dependencies:
@@ -109,6 +109,16 @@ postcss@^5.0.4, postcss@^5.2.15:
     source-map "^0.5.6"
     supports-color "^3.2.3"
 
+postcss@^5.2.18:
+  version "5.2.18"
+  resolved "https://registry.yarnpkg.com/postcss/-/postcss-5.2.18.tgz#badfa1497d46244f6390f58b319830d9107853c5"
+  integrity sha512-zrUjRRe1bpXKsX1qAJNJjqZViErVuyEkMTRrwu4ud4sbTtIBRmtaYDrHmcGgmrbsW3MHfmtIf+vJumgQn+PrXg==
+  dependencies:
+    chalk "^1.1.3"
+    js-base64 "^2.1.9"
+    source-map "^0.5.6"
+    supports-color "^3.2.3"
+
 regenerate@^1.2.1:
   version "1.3.2"
   resolved "https://registry.yarnpkg.com/regenerate/-/regenerate-1.3.2.tgz#d1941c67bad437e1be76433add5b385f95b19260"