set flag to exit script if any command fails
set flag to exit script if any command fails

When running the `script/bootstrap` command on a Ruby version that is
not supported yet (2.2.0), the script experienced a failure during the
first step, trying to install eventmachine 1.0.3.

The eventmachine gem has since been updated to support building on Ruby
2.2.0 (See https://github.com/eventmachine/eventmachine/pull/503 for
details), however the bootstrap script itself should have stopped at
that point, surfacing the error to the operator to examine.

pkdevbox/github-services,galeksandrp/github-services,headius/github-services,wassemgtk/github-services,github/github-services,Zarthus/github-services,brntbeer/github-services,galeksandrp/github-services,parshap/github-services,Mumakil/github-services,Mumakil/github-services,timabbott/github-services,adrianschroeter/github-services,parshap/github-services,supriyantomaftuh/github-services,supriyantomaftuh/github-services,YOTOV-LIMITED/github-services,timabbott/github-services,Zarthus/github-services,adrianschroeter/github-services,dataRonin/github-services,wassemgtk/github-services,VladRassokhin/github-services,VladRassokhin/github-services,illan/github-services,github/github-services,brntbeer/github-services,illan/github-services,YOTOV-LIMITED/github-services,headius/github-services,Jdesk/github-services,dataRonin/github-services,pkdevbox/github-services,Jdesk/github-services