escaping $ in NMI_FAIL query

post all now reads:
```
#!/bin/bash                                                                                                                                           
# polyphemus post_all callbacks                                                                                                                       
if [ -z $_NMI_STEP_FAILED ]
then
    curl --form status='{"status":"success","number":694,"description":"build and test completed successfully"}' http://cyclus-ci.fuelcycle.org:80/ba\
tlabstatus
else
    curl --form status='{"status":"failure","number":694,"description":"build and test failed"}' http://cyclus-ci.fuelcycle.org:80/batlabstatus
fi
```