require less >= 1.7.5

Fixes bug where `vbox > *` style was applied to `vbox()` classes.

The churn we were seeing on update to less 1.7.5 was not due to a bug, but due to *fixing* a bug. Everywhere we had `.vbox()`, both the `.vbox` style and the `.vbox > *` style were being applied. less 1.7.5 fixes this, so is required from now on.