Fix test_elliptic test failure

 * On master
```bash
PYTHONHASHSEED=1421912842 bin/test sympy/mpmath/tests/test_elliptic.py -k test_sn_cn_dn_identities --seed 1970464
============================= test process starts ==============================
executable:         /home/ptb/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       gmpy 1.17
random seed:        1970464
hash randomization: on (PYTHONHASHSEED=1421912842)

sympy/mpmath/tests/test_elliptic.py[1] F                                  [FAIL]

________________________________________________________________________________
_________ sympy/mpmath/tests/test_elliptic.py:test_sn_cn_dn_identities _________
  File "/home/ptb/gitrepos/sympy/sympy/mpmath/tests/test_elliptic.py", line 556, in test_sn_cn_dn_identities
    assert(equality.ae(0))
AssertionError

============= tests finished: 0 passed, 1 failed, in 0.10 seconds ==============
DO *NOT* COMMIT!
```

Following the guidance in #5821, add epsilon to ensure values are correct to within 1e-99 rather than exact

* this PR
```bash
PYTHONHASHSEED=1421912842 bin/test sympy/mpmath/tests/test_elliptic.py -k test_sn_cn_dn_identities --seed 1970464
============================= test process starts ==============================
executable:         /home/ptb/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       gmpy 1.17
random seed:        1970464
hash randomization: on (PYTHONHASHSEED=1421912842)

sympy/mpmath/tests/test_elliptic.py[1] .                                    [OK]

================== tests finished: 1 passed, in 0.11 seconds ===================
```