upgrade debugger to version ~> 1.6.8

Open PRs that triggered Travis recently broke because Travis upgraded their defaults, so debugger was not a good version.
https://github.com/AgileVentures/LocalSupport/pull/69
```
it had rvm 1.25.25 now 1.25.28
it had ruby-1.9.3-p545 now p547
it had rubygems 2.2.2 now 2.3.0
it had bundler 1.6.1 now 1.6.2
```
Update to version ~> 1.6.8 will freshen previous 1.6.6