Fix update script test for fetch when releasing a new version

Previously we were relying on VERSION.txt for the name of the latest
release, however this doesn't work in scenarios where we have changed
the version number in preperation but have not yet made the release, as
we found recently (see issue #1205).

Instead we get the latest release version using the GitHub API [[1]].
Note that we have to drop the `v` at the beginning of the tag name,
because for some reason GitHub doesn't include that in the archive file
name.

This commit uses the `superagent` library to do the request to avoid a
lot of boilerplate; note that we have to set the user agent to
something, because otherwise GitHub will forbid the request [[2]].

[1]: #1205 (comment)
[2]: ladjs/superagent#1495 (comment)

__tests__/spec/update-script.js

@@ -4,6 +4,7 @@ const fs = require('fs')
 const os = require('os')
 const path = require('path')
 const process = require('process')
+const request = require('superagent')
 
 /*
  * Constants
@@ -12,12 +13,6 @@ const process = require('process')
 const repoDir = path.resolve(__dirname, '..', '..')
 const script = path.join(repoDir, 'update.sh')
 
-const latestReleaseVersion = fs.readFileSync(
-  path.join(repoDir, 'VERSION.txt'), { encoding: 'utf8' }
-).trim()
-const latestReleaseBasename = `govuk-prototype-kit-${latestReleaseVersion}`
-const latestReleaseArchiveFilename = `${latestReleaseBasename}.zip`
-
 const headReleaseVersion = child_process.execSync(
   'git rev-parse HEAD', { encoding: 'utf8' }
 ).trim()
@@ -306,7 +301,17 @@ describe('update.sh', () => {
   })
 
   describe('fetch', () => {
-    it('downloads the latest release of the prototype kit into the update folder', () => {
+    it('downloads the latest release of the prototype kit into the update folder', async () => {
+      // check what GitHub thinks the latest release archive is
+      var response = await request
+        .get('https://api.github.com/repos/alphagov/govuk-prototype-kit/releases/latest')
+        .set('user-agent', 'node-superagent (tests for govuk-prototype-kit)')
+
+      if (response.error) throw response.error
+      const latestRelease = response.body
+      const latestReleaseVersion = latestRelease.tag_name.trim().slice(1) // need to drop the prefix 'v'
+      const latestReleaseArchiveFilename = `govuk-prototype-kit-${latestReleaseVersion}.zip`
+
       const testDir = 'fetch'
       fs.mkdirSync(path.join(testDir, 'update'), { recursive: true })