Update .bundle/config syntax to work across platforms

`BUNDLE_FORCE_RUBY_PLATFORM` `true` should force `nokogiri` being build
via source. This seems wasteful given Bundler should be able to use the
appropriate platform for it depending on the machine it's running from,
but that hasn't been the case for me recently, see
#385.

Without this setting, I am unable to `bundle exec rake new_release`.

Something is on my mind, though: Why does `rm -rf vendor/bundle &&
bundle install` work on my machine but the `bundle install` run from the
`new_release` Rake task fails?

Author: mokagio

.bundle/config

@@ -1,4 +1,5 @@
 ---
 BUNDLE_PATH: "vendor/bundle"
+BUNDLE_FORCE_RUBY_PLATFORM: "true"
 BUNDLE_SPECIFIC_PLATFORM: "false"
 BUNDLE_JOBS: "16"