Return last builds on each branch for /builds?branches=true

This is a hack to fix travis-ci/travis-web#123 easier. A proper solution
would be to refactor how /branches work.

lib/travis/api/app/endpoint/builds.rb

@@ -4,7 +4,8 @@ class Travis::Api::App
   class Endpoint
     class Builds < Endpoint
       get '/' do
-        respond_with service(:find_builds, params)
+        name = params[:branches] ? :find_branches : :find_builds
+        respond_with service(name, params)
       end
 
       get '/:id' do

spec/integration/v2/builds_spec.rb

@@ -29,4 +29,9 @@ describe 'Builds' do
     response = get "/repos/svenfuchs/minimal/builds/#{build.id}", {}, headers
     response.should deliver_json_for(build, version: 'v2')
   end
+
+  it 'GET /builds/1?repository_id=1&branches=true' do
+    response = get "/builds?repository_id=#{repo.id}&branches=true", {}, headers
+    response.should deliver_json_for(repo.last_finished_builds_by_branches, version: 'v2')
+  end
 end