Move uv_default_loop() call back to top of _julia_init().

Tested early calls to jl_errorf() and jl_exit().
As long as no uv streams are in the uv_shutdown_queue when jl_exit() is
called it will not go into the while(item) loop and will not use JL_TRY.
Therefore the dependancy of JL_TRY on jl_init_tasks() does not cause a
problem since the JL_STD* are not re-initialised to uv_stream types
until after jl_init_tasks().