Revert "Update gem bundle for Jekyll 3.2"

[hsbt]

@stomar I found site generation error with jekyll 3.2.0. see following log:

remote:        Building jekyll site
remote:        Configuration file: /tmp/build_5d2a93ebd3bc47dec966ddd865c8af4a/_config.yml
remote:        Source: /tmp/build_5d2a93ebd3bc47dec966ddd865c8af4a
remote:        Destination: /tmp/build_5d2a93ebd3bc47dec966ddd865c8af4a/_site
remote:        Generating site...
remote:        rake aborted!
remote:        Errno::ENOENT: No such file or directory @ utime_internal - /tmp/build_5d2a93ebd3bc47dec966ddd865c8af4a/_site/bin/erb
remote:        /tmp/build_5d2a93ebd3bc47dec966ddd865c8af4a/vendor/bundle/ruby/2.3.0/gems/jekyll-3.2.0/lib/jekyll/static_file.rb:149:in `utime'

It seems heroku specific error on jekyll 3.2.0 probably( so my osx env is fine ). we should temporary downgrade it.

[stomar]

Strange... worked fine on local machine and also Travis.