fix(netlify build): install needed bundler

Author: ff6347

netlify.toml

@@ -1,3 +1,3 @@
 [build]
   publish = "_site/"
-  command = "gem install bundler -v '2.1.4' && npm run build:api-markup && npm run webpack:prod && jekyll build"
+  command = "gem install bundler -v '2.1.4' && npm run build:api-markup && npm run webpack:prod && bundle exec jekyll build"