Write new Gemfile.lock update task/script

C:\Users\xmr\Desktop\bootstrap>node -v && npm -v
v0.12.7
2.11.3

C:\Users\xmr\Desktop\bootstrap>grunt update-gemfile-lock
Running "exec:bundleUpdate" (exec) task
Fetching gem metadata from https://rubygems.org/...........
Fetching version metadata from https://rubygems.org/...
Fetching dependency metadata from https://rubygems.org/..
Resolving dependencies...
Using blankslate 2.1.2.4
Using hitimes 1.2.2
Using timers 4.0.2
Using celluloid 0.16.1
Using fast-stemmer 1.0.2
Using classifier-reborn 2.0.3
Using coffee-script-source 1.9.1.1
Using execjs 2.6.0
Using coffee-script 2.4.1
Using colorator 0.1
Using ffi 1.9.10
Using jekyll-coffeescript 1.0.1
Using jekyll-gist 1.3.3
Using jekyll-paginate 1.1.0
Using sass 3.4.17
Using jekyll-sass-converter 1.3.0
Using rb-fsevent 0.9.5
Using rb-inotify 0.9.5
Using listen 2.10.1
Using jekyll-watch 1.2.1
Using kramdown 1.8.0
Using liquid 2.6.3
Using mercenary 0.3.5
Using posix-spawn 0.3.11
Using yajl-ruby 1.2.1
Using pygments.rb 0.6.3
Using redcarpet 3.3.2
Using safe_yaml 1.0.4
Using parslet 1.5.0
Using toml 0.1.2
Using jekyll 2.5.3
Using jekyll-redirect-from 0.8.0
Using rainbow 2.0.0
Using rouge 1.9.1
Using scss-lint 0.38.0
Using bundler 1.10.6
Bundle updated!
>> 'BUNDLE_GEMFILE' is not recognized as an internal or external command,
>> operable program or batch file.
>> Exited with code: 1.
Warning: Task "exec:bundleUpdate" failed. Use --force to continue.

Aborted due to warnings.


Execution Time (2015-08-25 06:02:22 UTC)
exec:bundleUpdate  7.5s  ███████████████████████████████████████████ 100%
Total 7.5s

It seems it's caused by

...
return 'BUNDLE_GEMFILE=' + gemfile + ' bundle update';