fix bash version check

The current rvm-installer accepts unsupported bash versions(3.2.3 ~ 3.2.9) due to the incorrect version comparison.

ex) The following code which is extracted from rvm-installer does not show the error message.

```
BASH_VERSION="3.2.3"  # Set unsupported bash version for debug
BASH_MIN_VERSION="3.2.25"
if
	[[ -n "${BASH_VERSION:-}" &&
	"$(\printf "%b" "${BASH_VERSION:-}\n${BASH_MIN_VERSION}\n" | LC_ALL=C \sort -n -t"." | \head -n1)" != "${BASH_MIN_VERSION}"
	]]
then
	echo "BASH ${BASH_MIN_VERSION} required (you have $BASH_VERSION)"
	exit 1
fi
```

I've fixed it by changing the sort option.