README: style updates (#3337)
README: style updates (#3337)

Improved some stylistic issues.

* Updated Travis CI reference to CircleCI.
* Switched to CircleCI "shield" style badge to fit in better with the other badges.
apletnev/koreader,houqp/koreader,NiLuJe/koreader,mwoz123/koreader,koreader/koreader,poire-z/koreader,mihailim/koreader,poire-z/koreader,NiLuJe/koreader,koreader/koreader,pazos/koreader,lgeek/koreader,Frenzie/koreader,Markismus/koreader,Frenzie/koreader,Hzj-jie/koreader