Don't set TEST_DOCTESTS when it's false

This makes the Travis build matrix much easier to read.
Try an empty env
Corrects the Differential Extension:

     The following PR corrects the differential extension of the test
case in risch.py under test_recognize_log_derivative. It changes
DE = DifferentialExtension(extension={'D': [Poly(1, x), Poly(1, t)]}) to
DE = DifferentialExtension(extension={'D': [Poly(1, x)]}) and also adds
the case where DE = DifferentialExtension(extension={'D': [Poly(1, x), Poly(t**2 + 1, t]})
Merge pull request #2384 from cheatiiit/correct_predict

DE in test_risch.py (test_recognize_log_derivative)
Merge pull request #2373 from asmeurer/travis

Don't set TEST_DOCTESTS when it's false
Fix up mechanics printer.
Add test for MechanicsLatexPrinter.
Use _print on exponent.
Remove trailing newline.
Merge pull request #2392 from stefanv/fix_mechanics_printer

Fix up mechanics printer.
Add Stefan van der Walt to AUTHORS/aboutus. Welcome to SymPy!