Removed empty inversion.heuristic

It was causing warnings in the doc build. I'll turn on doc building in
Travis CI and make the warnings into errors. So this had to go.
Doc corrections in mesher

Doctests in mesher.Tesseroid were unindented causing an error in the
doc build.
Fixed Travis config for doc building

Added rule to clean the build binaries before building the docs and make
warnings into errors