Fix backline package name

Without this patch the existing backline packages are missing encoding
translators which is a problem because it's interfering with pry's
ability to display interactive prompts.  This in turn is making it
really difficult to build crossfader since there's a problem I'm having
difficult debugging because pry doesn't work right.

This patch addresses the problem by updating the backline configuration
to use the new build names.  This should result in a backline package
with bundler installed and all of the encodings properly generated.
Fix rake task unable to execute commands

Without this patch we're seeing failures we weren't previously seeing in
the Jenkins builds.  The failures look like:

    $ bundle exec rake crossfader --trace
    ** Invoke crossfader (first_time)
    ** Execute crossfader
    rm -rf pkg
    rm -rf destroot
    rm -rf artifacts
    git clean -fdx src/
    git checkout HEAD src/
    rm -rf /opt/crossfader/runtime
    bundle exec rake PVM_CONFIG=crossfaderuntime build
    rake aborted!
    Command failed with status (127): [bundle exec rake PVM_CONFIG=crossfaderunti...]
    /Users/jenkins/workspace/Crossfader-Package-MacOSX-10.8/.bundle/gems/ruby/1.9.1/gems/rake-10.1.0/lib/rake/file_utils.rb:55:in `block in create_shell_runner'
    /Users/jenkins/workspace/Crossfader-Package-MacOSX-10.8/.bundle/gems/ruby/1.9.1/gems/rake-10.1.0/lib/rake/file_utils.rb:45:in `call'
    /Users/jenkins/workspace/Crossfader-Package-MacOSX-10.8/.bundle/gems/ruby/1.9.1/gems/rake-10.1.0/lib/rake/file_utils.rb:45:in `sh'
    /Users/jenkins/workspace/Crossfader-Package-MacOSX-10.8/.bundle/gems/ruby/1.9.1/gems/rake-10.1.0/lib/rake/file_utils_ext.rb:37:in `sh'

I believe this is caused by a change in behavior where by the shell command
being executed is passed as a single string and as such the environment
variables for GEM_HOME and GEM_PATH are not being exported correctly.  This
patch addresses the problem by passing the command to execute as an array,
thereby avoiding the shell since the sh command follows the same semantics as
Kernel.exec()
Install crossfader script and package in the gems

Without this patch we don't actually have the crossfader command line
utility to manage GEM_HOME and GEM_PATH.  This patch addresses the
problem by installing the script from the crossfader gem directory.
Add minimally functional crossfader command