chore(docs.angularjs.org): install firebase dependencies before deploying

Firebase is trying to execute our functions code locally in order to
parse the triggers. Install npm dependencies to avoid errors like:

```
Error: Error parsing triggers: Cannot find module 'firebase-functions'
```

Closes #16453

scripts/docs.angularjs.org-firebase/readme.firebase.docs.md

@@ -20,4 +20,4 @@ build steps.
   Creates a server at localhost:5000 that serves from deploy/docs and uses the local function
 
 See /scripts/code.angularjs.org-firebase/readme.firebase.code.md for the firebase deployment to
-code.angularjs.org
+code.angularjs.org

scripts/travis/build.sh

@@ -2,6 +2,9 @@
 
 set -e
 
+readonly THIS_DIR=$(cd $(dirname $0); pwd)
+readonly ROOT_DIR="$THIS_DIR/../.."
+
 export BROWSER_STACK_ACCESS_KEY
 export SAUCE_ACCESS_KEY
 
@@ -80,6 +83,14 @@ case "$JOB" in
 
     if [[ "$DEPLOY_DOCS" == true || "$DEPLOY_CODE" == true ]]; then
       grunt prepareDeploy
+
+      if [[ "$DEPLOY_DOCS" == true ]]; then
+        # Install npm dependencies for Firebase functions.
+        (
+          cd "$ROOT_DIR/scripts/docs.angularjs.org-firebase/functions"
+          npm install
+        )
+      fi
     else
       echo "Skipping deployment build because conditions have not been met."
     fi
@@ -94,4 +105,4 @@ case "$JOB" in
       or\
       'deploy'."
     ;;
-esac
+esac