Make bashrc use PERLBREW_ROOT at installation time

If the generated bashrc file includes (by default) the value of
PERLBREW_ROOT at install time, the user doesn't have to remember to add
a line in bashrc (or wherever) to also set PERLBREW_ROOT again.
Merge pull request #359 from mmcclimon/inherit_root

Make bashrc use PERLBREW_ROOT at installation time