Use `env` to start tclsh instead of /bin/bash

This patch replaces the shebang line using /bin/bash with /usr/bin/env.
The previous version obfuscates which shell is being used by the tcl
script. Any benefits of using Bash as a start up are provided by `env`.

This closes issue #537.
Merge pull request #541 from romanofski/master

Use `env` to start tclsh instead of /bin/bash