✨ Update analytics vendor split experiments config

[jonathantyng-amp]

• Add gulp generate-vendor-configs to experiments command so that vendor JSONs are generated during the experiment build
• Enable --compile_vendor_configs by default, so no need to add this flag anymore

[jonathantyng-amp]

cc @zhouyx for review :)

cc @erwinmombay just want to make sure, is running 2 commands during experiment builds okay? e.g. gulp generate-vendor-jsons && gulp dist --defineExperimentConstant=ANALYTICS_VENDOR_SPLIT

[zhouyx]

A few notes on this PR:

1. Our release script always run gulp generate-vendor-jsons and gulp dist --compile_vendor_configs.
   By adding gulp generate-vendor-jsons to the experiment build command, we will be generating vendor jsons twice. But that should be fine given it only take a few seconds.
   Future idea is that we want to remove the gulp generate-vendor-jsons task from everywhere after launching the experiment.

2. Remove compile_vendor_configs in this PR. Because the flag itself is easy to be missed. Given in the future all build will need to copy the json file to the dist folder. we decide to remove the flag and make it a default process during build.

3. The reason we are making the change now is because we want to unblock @estherkim's work on integration test. We figure it's easier to modify the experiment build command instead of rewriting the default build command in travis.

[zhouyx]

@jonathantyng-amp could you please confirm that the build works even if gulp generate-vendor-jsons hasn't been run (no such json files to copy). In that case what will happen? Will the analytics-vendors folder be created? Thanks.

[jonathantyng-amp]

Tested locally and confirmed that build works with no JSON files to copy. If there are no vendor JSONs, dist/analytics-vendors folder will not be created

[zhouyx]

Great! Thanks