Default to rbzip, but use bzip2-ruby if available.

If bzip2-ruby is available, use it because it's faster.  Default to rbzip2 for compatibility.

tassadar [bzip2-ruby*] → time rake > /dev/null
rake > /dev/null  0.81s user 0.08s system 99% cpu 0.897 total

tassadar [bzip2-ruby*] → time RBZIP2=true rake > /dev/null
RBZIP2=true rake > /dev/null  1.34s user 0.08s system 99% cpu 1.436 total