some configurations in Travis skip optional dependencies,
which causes the doctests that rely on them to fail.

This fixes that by checking for those dependencies and skipping
the affected doctests with :skipif:

doctests updated for new 9-digit Quantity().__repr__()

added dependency on Serialize to requirements_docs.txt

in systems.rst: three errors remain, one may be a regression,
the others I'm not qualified to comment on

in nonmult.rst: some issues around floating point estimation are made
more robust by using print()

seems like Quantity().__repr__() changed since these
tests were written, and could benefit from some floating-point
truncation

in pitheorem.rst the only error was dictionary ordering, which
is now deterministic in python3.7, but may still cause
issues down the line

no doctests in getting.rst, marked all as code blocks

Few errors with respect to missing/wrong output in doctests were also fixed.

Closes #109