Update README to remove GWT and discuss java 8 requirement.
Update README to remove GWT and discuss java 8 requirement.

Review: da39a3ee5e6b4b0d3255bfef95601890afd80709@DanielSank

labrad/scalabrad-web,labrad/scalabrad-web,labrad/scalabrad-web,labrad/scalabrad-web,labrad/scalabrad-web