Replace postversion script with direct call to create-github-release.js

`yarn version` does not run the "postversion" script when called with
`--no-git-tag-version`.

Jenkinsfile

@@ -145,6 +145,7 @@ node {
 
                 // Bump the package version and create the GitHub release.
                 sh "yarn version --no-git-tag-version --new-version ${newPkgVersion}"
+                sh "scripts/create-github-release.js"
 
                 // Publish the updated package to the npm registry.
                 // Use `npm` rather than `yarn` for publishing.

package.json

@@ -148,7 +148,6 @@
     "test": "gulp test",
     "report-coverage": "codecov -f coverage/coverage-final.json",
     "version": "make clean build/manifest.json",
-    "postversion": "./scripts/postversion.sh",
     "prepublish": "yarn run build"
   }
 }

scripts/postversion.sh

-#!/bin/sh
-
-set -eu
-
-cd "$(dirname "$0")"
-
-# Skip GitHub release creation for QA releases.
-is_prerelease=$(node -p "require('../package.json').version.includes('-')")
-if [ $is_prerelease = "true" ]; then
-  echo "Skipping GitHub release creation for pre-release"
-  exit 0
-fi
-
-./create-github-release.js