dup 193 to 200
compile compatible to ruby 2.0

code from ruby-debug-base19x-0.11.30.pre11 .
fix ruby 2.0.0 error. undefined symbol: ruby_current_thread
Fix bug on ruby 2.0.0-p0.

On ruby 1.9.3-xxx GET_THREAD is a macro, on ruby 2.0.0-p0 is a function. <ruby-ver>/vm_core.h
Call rb_thread_current() before use GET_THREAD.
add 2.0.0 to .travis.yml
clearer error message for unsupported ruby versions
fix rvm/rbenv detection for 1.9* headers
until travis actually uses 2.0.0-p0 and tests pass
thanks @windwiny
Bump to version 1.4.0
mention 2.0.0 caveat