Move custom rf class outside the combsimp definition

This fixes, or rather, works around, a bizarre caching issue involving classes
defined inside functions.  See the discussion on pull request #1710.
change rf to _rf in simplify
Merge pull request #58 from smichr/rf

change rf to _rf in simplify
Added regression test for issue 7425.

Make sure lambdify can handle Python reserved keywords as symbol names.
Add PyPI shields to README
Merge pull request #7427 from moorepants/issue-7425

Added regression test for issue 7425.
Add feature to notify about travis builds on gitter
Merge pull request #1959 from asmeurer/rf

Move custom rf class outside the combsimp definition
Change on_success to always
Revert on_success back to never
Merge pull request #7434 from skirpichev/pypi-badgets

Add PyPI shields to README
Merge pull request #7435 from debugger22/travis_gitter_webhook

Add feature to notify about travis builds on Gitter
Unbundle mpmath: Kill mpmath source tree
Unbundle mpmath: Drop mpmath documentation
Unbundle mpmath: Correct list of modules and tests in setup.py
Unbundle mpmath: Fix imports
Unbundle mpmath: install python-mpmath
Unbundle mpmath: drop adapt_path.py script
Unbundle mpmath: drop workarrounds in bin/coverage_*
Unbundle mpmath: drop mpmath from blacklist of runtests.py