fix pypy travis, cache pip
fix pypy travis, cache pip

Matplotlib has numpy as setup requirement. Pip defers installing packages until
all packages are built. When building matplotlib, numpy is not yet installed
and setuptools(?) downloads and builds a potentially different version of numpy
than pip has already downloaded. This patch makes sure numpy is installed
before matplotlib is being built.

wijnandhoitinga/nutils,joostvanzwieten/nutils