drop dirname in favor of native Bash strings (1 of 2)

Convert `var=${dirname $filename)` to `var="${filename%/*}` in cases where there is no ambiguity.

Make sure that the path in `$BASH_IT` is absolute because this path gets embedded in the template `.bash_profile` file if selected by the user.

install.sh

@@ -184,7 +184,7 @@ if [[ $no_modify_config ]] && [[ $append_to_config ]]; then
 	exit 1
 fi
 
-BASH_IT="$(cd "$(dirname "$0")" && pwd)"
+BASH_IT="$(cd "${BASH_SOURCE%/*}" && pwd)"
 
 case $OSTYPE in
 	darwin*)