Pin Boule to < 0.6 for the next release

[leouieda]

The IGRF PR #556 introduced Boule as a dependency. But Boule v0.6 (fatiando/boule#232) is introducing some backwards incompatible changes. We should add a guard here to limit to <0.6 since this version of Harmonica is not compatible with the older Boule. We can make a new release after Boule is out.