Fix chromedriver

Related to !1075 (merged)

Previously, the chromedriver binary was stored directly in the downloaded chromedriver zip. However, it appears that this has changed and now the zip contains a directory named chromedriver-linux64, within which the chromedriver binary is stored. Unzipping this zip resulted in the binary being placed in /usr/local/bin/chromedriver-linux64/chromedriver, which doesn't work. This fixes the problem by unzipping the binary directly as /usr/local/bin/chromedriver.

Closes #704 (closed)