Upgrade asteval to 0.9.1

asteval 0.9 requires numpy, and if it's not installed, the build will fail (nor will it attempt to install it as a dependency).  This PR updates asteval to an as-yet-unreleased version (built from https://github.com/newville/asteval/commit/1400a9215e4094dac19f557bc1af545f0c36b7c8) that doesn't require numpy.