Update readme for 0.8.0 release

To be merged once the play-slick 0.8.0 artifacts hit Maven Central. [They don't seem to be there just yet.](http://search.maven.org/#search%7Cga%7C1%7Cg%3A%22com.typesafe.play%22%20AND%20(a%3A%22play-slick_2.10%22%20OR%20a%3A%22play-slick_2.11%22))