Correctly provide default value for DRIP_POOL

This syntax matches all other default values.  Previously this failed
with:

/home/gaul/bin/drip: line 223: ((: 0 <  : syntax error: operand expected (error token is "<  ")
Merge pull request #82 from andrewgaul/drip-pool-default

Correctly provide default value for DRIP_POOL