Remove .sh extension from provision script

In Contributing.md, it mentions that VVV follows [Google's shell style guide](https://google-styleguide.googlecode.com/svn/trunk/shell.xml#File_Extensions). 

If so, we should remove the `.sh` extension from our scripts, mainly `provision.sh`, as its preferred to keep them without an extension. "Executables should have no extension (strongly preferred)"

This PR renames it, and updates all references to the script as well. I've left database/import-sql.sh for now, and we can also rename that if you want.