Change default_branch default value after qiskit migrates to use main

[mtreinish]

As pointed out in: https://github.com/Qiskit/qiskit-bot/pull/9#pullrequestreview-636035677 a better forward looking default value for the default_branch config options added in #9 is main. Once the qiskit repos currently using qiskit-bot (qiskit/qiskit, terra, aer, ignis, aqua, nature, optimization, finance, and machine-learning) all migrate to use main for their default branch we should change the default value.