Fix elixir support for inline do: keywords

I came across an issue with Elixir again, when using inline `do: ` keyword arguments:

    for x <- 1..10, do: x|

    # hit enter

    for x <- 1..10, do: x
    |
    end

So we must also check that the `do` is not immediately followed by a colon (a space in between would be a syntax error, so it's not something to consider). Alternatively, we could check whether there are only whitespace (or no) characters until the end of the line, which is typically the case. This PR fixes it with a negative lookahead after `do`.