workaround for radiasoft/ops#324: numpy==1.15.4
workaround for radiasoft/ops#324: numpy==1.15.4

There's some incorrect code in warp that looks at numpy
to see if a particular module's file ends with `.so`
to determine if modules can be dynamically loaded. This
code has to change, but it's too complex so hardwire
numpy to 1.15.4 which is a known working version

radiasoft/download,radiasoft/download