Fix end of Ruby line detection

Should fix the problems in #41. Regex is currently excluding the last character in the line from the embedded ruby scope (unless there's a block). This fixes that by using a negative lookbehind, and also simplifies the first part of the expression that doesn't seem to have any purpose except for mangling scope (it's in the HAML bundle as well so I assume it was taken from there).

This still does not apply Ruby syntax scope to the next line if a line ends on a continuation character (`,` or `\`), that looks to be a more complicated fix. However if your color scheme has a different background color for embedded Ruby, it should continue right to the end of the original line, which at least gives some indication.