Hotfix: moves re-exports of display_violinplot into the try-except-else guard

[ilumsden]

Fixes #158

To prevent issues with importing the optional seaborn dependency, all re-exports of "display" modules need to be placed in the else statement of the "try-except-else" guard in thicket/stats/__init__.py. However, the re-exports for display_violinplot.py were not placed in that else statement, which caused the import issues in #158.

This PR fixes this.