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
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
Merge pull request #8123 from pbrady/remove_extremely_slow_tests

Skip the slowest of the slow tests on travis