Skip the test in test_failing_integrals that is too slow for @slow

This test never finishes (or at least, it would probably take days), so
without a timeout, the slow tests don't complete. This was causing the slow
tests to timeout in the NiPy buildbot
(http://nipy.bic.berkeley.edu/builders/sympy-py2.6-32/builds/0/steps/shell_4/logs/stdio).
Merge pull request #2424 from asmeurer/really_slow

Skip the test in test_failing_integrals that is too slow for @slow
Bug fixes proposed by pernici
fixed ``sqrt_mod`` for negative modulus
Merge pull request #2430 from pernici/pr3

fixed ``sqrt_mod`` for negative modulus
Merge branch 'master' into Diophantine_Equation_Module_II
A bug fixed and tests updated
Merge branch 'Diophantine_Equation_Module_II' of https://github.com/thilinarmtb/sympy into Diophantine_Equation_Module_II
Becuase the Two versions are different.