Fix broken links in README.md
Fix broken links in README.md

- Corrected href to "the blog extension guide"
- Corrected image src and href for Travis CI build status
LessEverything/middleman-blog,sahat/middleman-blog,fukayatsu/middleman-blog,sahat/middleman-blog,evaryont/middleman-blog,MinasMazar/middleman-blog,evaryont/middleman-blog,middleman/middleman-blog,evaryont/middleman-blog,5t111111/middleman-blog,fukayatsu/middleman-blog,sahat/middleman-blog,MinasMazar/middleman-blog,middleman/middleman-blog,fukayatsu/middleman-blog,dhulihan/middleman-blog,middleman/middleman-blog,dhulihan/middleman-blog,LessEverything/middleman-blog,5t111111/middleman-blog,5t111111/middleman-blog,dhulihan/middleman-blog,MinasMazar/middleman-blog