Fix boolalg test failure

See issue #7684.  The failure is caused by non-deterministic (hash) ordering of _args.

This PR orders _args in a consistent and deterministic manner (using the str based comparison in `ordered`).

* on master
```bash
PYTHONHASHSEED=2264035752 bin/test sympy/logic/tests/test_boolalg.py --seed 52767090 -k test_Xor
============================= test process starts ==============================
executable:         /home/ptb/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
random seed:        52767090
hash randomization: on (PYTHONHASHSEED=2264035752)

sympy/logic/tests/test_boolalg.py[1] F                                    [FAIL]

________________________________________________________________________________
__________________ sympy/logic/tests/test_boolalg.py:test_Xor __________________
  File "/home/ptb/gitrepos/sympy/sympy/logic/tests/test_boolalg.py", line 71, in test_Xor
    assert Xor(True, False, False, A, B) == ~Xor(A, B)
AssertionError

============= tests finished: 0 passed, 1 failed, in 0.13 seconds ==============
DO *NOT* COMMIT!
```

* this PR
```bash
PYTHONHASHSEED=2264035752 bin/test sympy/logic/tests/test_boolalg.py --seed 52767090 -k test_Xor
============================= test process starts ==============================
executable:         /home/ptb/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
random seed:        52767090
hash randomization: on (PYTHONHASHSEED=2264035752)

sympy/logic/tests/test_boolalg.py[1] .                                      [OK]

================== tests finished: 1 passed, in 0.18 seconds ===================
```