chore(version-info): use branchPattern to check tag

We have two fields in package.json for checking the current version:

* branchVersion
* branchPattern

The `branchVersion` field is used to work out what version to use in the
docs application, so we should not update this to the most recent version
until that version is on the Google CDN. Otherwise the docs app will break.

The `branchPattern` is used to determine what branch we are currently
working from and is generally used as a gate-keeper to prevent invalid
releases from the wrong branch.

The `getTaggedVersion()` method was using the `branchVersion` to check
that the tagged commit was valid but this fails when we are moving to a
new minor version with release candidates.

This fix avoids the problem by doing a custom comparison against the
`branchPattern` instead.

Author: petebacondarwin

lib/versions/version-info.js

@@ -73,6 +73,11 @@ function getBuild() {
   return 'sha.' + hash;
 }
 
+function checkBranchPattern(version, branchPattern) {
+  // check that the version starts with the branch pattern minus its asterisk
+  // e.g. branchPattern = '1.6.*'; version = '1.6.0-rc.0' => '1.6.' === '1.6.'
+  return version.slice(0, branchPattern.length - 1) === branchPattern.replace('*', '');
+}
 
 /**
  * If the current commit is tagged as a version get that version
@@ -85,7 +90,7 @@ var getTaggedVersion = function() {
     var tag = gitTagResult.output.trim();
     var version = semver.parse(tag);
 
-    if (version && semver.satisfies(version, currentPackage.branchVersion)) {
+    if (version && checkBranchPattern(version, currentPackage.branchPattern)) {
       version.codeName = getCodeName(tag);
       version.full = version.version;
       version.branch = 'v' + currentPackage.branchPattern.replace('*', 'x');