add coveralls badge back to README
add coveralls badge back to README

alisaifee/holmium.core,tjlee/holmium.core,tjlee/holmium.core,kejkz/holmium.core,jlecount-sungevity/holmium.core,alisaifee/holmium.core,jlecount-sungevity/holmium.core,jlecount-sungevity/holmium.core,alisaifee/holmium.core,kejkz/holmium.core,kejkz/holmium.core,tjlee/holmium.core,kejkz/holmium.core,tjlee/holmium.core,alisaifee/holmium.core,jlecount-sungevity/holmium.core