Add travis badge to readme, remove jruby 1.8 and ree

I thought I had the syntax correct in my .travis.yml file to show that
it was okay to have failing builds in jruby 1.8 and ree, but apparently
I was incorrect. Let's see if this gets a passing build now and I'll
research the correct way to document that we're testing against jruby
1.8 and ree but expect failures anyway.