bashrc: switch C-w binding

Switch the binding of C-w from unix-word-rubout to backward-kill-word. This
means that C-w will now stop at punctuation inflections, and not just at
whitespace.

For example, if I were typing the following:

	$ bitbake core-image-minnimal

and wanted to edit that last part, previously typing C-w would delete all of
"core-image-minnimal" meaning I'd have to re-type "core-image-". With this new
binding C-w will now only erase "minnimal" allowing me to retype just that
last part.

The following was helpful in finding this solution:
	http://stackoverflow.com/questions/10980575/how-can-i-unbind-and-remap-c-w-in-bash
	http://www.gnu.org/software/bash/manual/html_node/Bindable-Readline-Commands.html

Signed-off-by: Trevor Woerner <twoerner@gmail.com>