Allow 2 decimals for energy amount

[jbaron-mx]

This PR fixes a rounding problem caused by having the amount column defined as float(8,1), where if decreasing the energy by 0.25 for instance, it will be persisted to the database as 0.20

Issue can be recreated in Tinker as follows:

# Add energy of 1 to an energisable model
$hashtag->addEnergy(1);
$hashtag->energy->amount; // 1.0

# Decrease energy by 0.25
$hashtag->decayEnergy(0.25)
$hashtag->refresh(); 
$hashtag->energy->amount; // 8.0 instead of 0.75