Don't set TEST_DOCTESTS when it's false

This makes the Travis build matrix much easier to read.
Try an empty env
Test to check bug in dsolve
Explicitly define variables in the polys benchmark

This avoids an enormous number of pyflakes errors for this file.
Explicitly define variables in bench_groebnertools.py
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!
Merge pull request #2389 from asmeurer/polysbench

Explicitly define variables in the polys benchmark
Added @XFAIL test
Merge pull request #2379 from Manoj-Kumar-S/constant

Added XFAIL test for dsolve