Disable parallel builds at top level.

Sub-builds are still parallelized. This is a workaround for rustpkg
issues. See https://github.com/mozilla/rust/issues/9650
auto merge of #1292 : metajack/servo/not-parallel, r=larsbergstrom

Sub-builds are still parallelized. This is a workaround for rustpkg
issues. See https://github.com/mozilla/rust/issues/9650