feat!: update to jest 25

Jest has released version 25 under the `next` dist-tag:
- https://www.npmjs.com/package/jest/v/25.0.0
- jestjs/jest#8862

See breaking changes at
https://github.com/facebook/jest/blob/6c0a16ab2f88ea645c04880a5491b3a8c2dfc66b/CHANGELOG.md

The maintainer promised no more user facing breaking changes planned
jestjs/jest#8956 (comment)

Considering Vue CLI does not bump the major version often, we need to
use the latest possible version.

Currently there will be warnings from ts-jest.
Once this PR gets merged kulshekhar/ts-jest#1232
the warnings will be eliminated.

packages/@vue/cli-plugin-unit-jest/generator/index.js

@@ -54,7 +54,6 @@ module.exports = (api, _, __, invoking) => {
     if (api.hasPlugin('babel')) {
       api.extendPackage({
         devDependencies: {
-          'babel-jest': '^24.8.0',
           '@babel/core': '^7.4.5'
         }
       })

packages/@vue/cli-plugin-unit-jest/package.json

@@ -27,8 +27,8 @@
     "@babel/plugin-transform-modules-commonjs": "^7.4.4",
     "@vue/cli-shared-utils": "^4.0.0-rc.3",
     "babel-core": "7.0.0-bridge.0",
-    "babel-jest": "^24.8.0",
-    "jest": "^24.8.0",
+    "babel-jest": "^25.0.0",
+    "jest": "^25.0.0",
     "jest-serializer-vue": "^2.0.2",
     "jest-transform-stub": "^2.0.0",
     "jest-watch-typeahead": "^0.3.1",