build: reset version to 6.0.0-beta.0 in prep for 6.0.0-beta.1

ELECTRON_VERSION

@@ -1 +1 @@
-6.0.0-nightly.20190404
+6.0.0-beta.0

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "electron",
-  "version": "6.0.0-nightly.20190404",
+  "version": "6.0.0-beta.0",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {

package.json

@@ -1,6 +1,6 @@
 {
   "name": "electron",
-  "version": "6.0.0-nightly.20190404",
+  "version": "6.0.0-beta.0",
   "repository": "https://github.com/electron/electron",
   "description": "Build cross platform desktop apps with JavaScript, HTML, and CSS",
   "devDependencies": {

script/lib/version-utils.js

@@ -47,7 +47,7 @@ async function nextBeta (v) {
   tags.sort((t1, t2) => semver.gt(t1, t2))
 
   // increment the latest existing beta tag or start at beta.1 if it's a new beta line
-  return tags.length === 0 ? semver.inc(next, 'beta', 'prerelease') : semver.inc(tags.pop(), 'prerelease')
+  return tags.length === 0 ? `${next}-beta.1` : semver.inc(tags.pop(), 'prerelease')
 }
 
 async function getElectronVersion () {