Website: Fix aspect ratio of icons in the showcase on homepage
Website: Fix aspect ratio of icons in the showcase on homepage

Summary:
I ran the website locally and noticed the icons in the showcase on the homepage look squashed:

![screenshot 2017-03-07 22 28 23](https://cloud.githubusercontent.com/assets/346214/23681227/18fbec6e-0386-11e7-9658-93907cbb3337.png)

Made the icons consistent with /showcase.html:

https://github.com/facebook/react-native/blob/d54c7f82822f623a903858d8d686d7f14ec71aae/website/styles/_showcase.scss#L74
Closes https://github.com/facebook/react-native/pull/12773

Differential Revision: D4669767

Pulled By: hramos

fbshipit-source-id: be5a758dd912083fc02c8e0b56c30c8617dcf3df

ankitsinghania94/react-native,ankitsinghania94/react-native,ankitsinghania94/react-native,ankitsinghania94/react-native,ankitsinghania94/react-native,ankitsinghania94/react-native,ankitsinghania94/react-native,ankitsinghania94/react-native