Removed the slow tests which are never actually run to completion on
travis due to timeout errors.  Those tests and there run times are:

6745.05s call     sympy/physics/mechanics/tests/test_kane3.py::test_bicycle
2083.29s call     sympy/solvers/tests/test_ode.py::test_1st_exact2
1522.66s call     sympy/integrals/tests/test_failing_integrals.py::test_issue_4540
1221.33s call     sympy/core/tests/test_wester.py::test_W25
1090.84s call     sympy/integrals/tests/test_failing_integrals.py::test_issue_4941
963.31s call     sympy/solvers/tests/test_solvers.py::test_issue_6528
437.72s call     sympy/integrals/tests/test_heurisch.py::test_pmint_WrightOmega
411.27s call     sympy/core/tests/test_wester.py::test_W24
MatExpr: Fix entry when power is zero and other cases

 *  `MatPow(A, 0)[0, 0]` would give an error because an empty list was
    passed to MatMult.  Fixed, added tests.
 *  Entries of `MatPow` of ImmutableMatrix added as a special case.
    This fixes `.as_explicit` for MatPow of ImmutableMatrices, added
    tests for this.
 *  Previously, `MatPow._entry` would return None if couldn't do it,
    instead raise errors.
 *  Negative powers probably do-able in some cases, left comments
    for future improvement.
Implementing the Rademacher distribution
Only skip extremely slow tests if we are running on travis.

- The enviroment variable TRAVIS_BUILD_NUMBER is checked to determine if
we are running on travis.
- specifying timeout=180 is no long necessary in bin/test_travis.sh
Add --split option to pytest
Increase default cache size from 500 to 1000
Merge pull request #8198 from pbrady/large_cache

Increase default cache size from 500 to 1000
Merge pull request #8183 from pbrady/pytest_split

Add --split option to py.test
Merge pull request #8123 from pbrady/remove_extremely_slow_tests

Skip the slowest of the slow tests on travis
Merge pull request #8104 from pelegm/rademacher

Implementing the Rademacher distribution
Merge pull request #8137 from cbm755/matpow_entry

MatExpr: Fix entry when power is zero and other cases
Merge pull request #8201 from skirpichev/limit-dir-printing

Printing support for directional limits
Add support for bidirectional limits (dir="real")