Fix false "BUILD FAILED" message when installing Rubinius

Installing Rubinius would print "BUILD FAILED" at the `bundle` step but
the installation would proceed nevertheless and finish successfully.

It turns out that the `command ...` expression will trigger the ERR trap
on failure even when it's a part of `||` list or a condition inside an
`if` statement. This defies how the ERR trap is supposed to work, so the
workaround is to execute it in a subshell which doesn't inherit the ERR
trap of its parent.
Install the Rubinius bundle in isolation

Makes sure that installing Rubinius won't touch other gems outside the
build directory.
Merge branch 'rbx-build'