Updated vi-VN for some missing terms (#2097)

```yml
search_placeholder_text    : "Nhập từ khóa cần tìm..."
results_found              : "Kết quả tìm được"
back_to_top                : "Về đầu trang"
```

Update CHANGELOG and history

Fix French translation typo (#2096)

Update CHANGELOG and hsitory

Fix figures when they are inside of a list (#2094)

Update CHANGELOG and history

Replace jQuery Smooth Scroll with Smooth Scroll + Gumshoe (#2082)

* Replace jQuery Smooth Scroll with Smooth Scroll + Gumshoe
* Change smooth scrolling speed
* Set maximum smooth scroll duration
* Remove Font Awesome CSS Ppseudo elements attribute from JS script

Close #2050, close #2075

Add styling for GFM task lists

Close #2092

Adjust wording

Update CHANGELOG and history

Update Google Universal Analytics to load async (#2079)

Update CHANGELOG

Remove Google+

Google+ is being shutdown on April 2, 2019. Social sharing buttong, comment provider, and author link configs have been removed from the theme.

ref: https://support.google.com/plus/answer/9195133

Loosen Bundler dependency for theme gem

Update CHANGELOG and history

Add missing translation of chinese (#2072)

Add missing Chinese text strings

Release 4.15.2 💎

Update CHANGELOG and history

exit search screen with Esc key (#2055)

* exit search with esc key feature is added
* closing search screen with esc key feature is added to _main.js

Fix bad YAML

Remove comma