Update coala banner on README

[yukiisbored]

The banner on coala/coala has changed into a more modern look to match the new website. I think the same thing should be done as well on this repository for uniformity.

[yashasingh]

@yukiisbored I would like to work on this issue.

[yukiisbored]

@yashasingh Hey, head over to our gitter chat and ask @co-robo to assign yourself to this issue by doing corobo assign <issue link>. Have fun!