Utilized pathname to avoid problems with symlinks.

Deploy.rb will usually not be called from its actuall location, but from
a symlink. Using require with only __FILE__ will result in error.
Pathname allows us to get the real path of the file and require other
files relative to this.
Changed the way of setting env variables.

The setting of environment variables was made a task of its own. What
happened is that a process was spawned by ruby and executed the EXPORT
bash command. However, this only changed the environment of the child
process and not the parent. Any further executed commands would still
only run with the old and previous environment.
Refactored the execution of task.

The execution of a task is part of the Tasks module.
Removed version number from shebang.