Do not append a newline char to @line ivar

This commit indirectly fixes issue #688 (Coolline repeats the last two
characters of closing blocks) from the bug tracker of `pry`:
pry/pry#688

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Description by jasonLaster:

When correcting for indentation, the last 2 characters of the closing
keyword "end" are repeated. Another weird glitch is that this doesn't
happen with Readline.

  [1] pry(main)> loop do
  [1] pry(main)*   loop do
  [1] pry(main)*   endnd
  [1] pry(main)* endnd
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To be honest, I found out that by accident. I don't fully realize why
this commit fixes the bug, but I think I found a mare's nest.

Signed-off-by: Kyrylo Silin <kyrylosilin@gmail.com>
Reported-by: Jason Laster <jason.laster.11@gmail.com>

lib/coolline/coolline.rb

@@ -252,7 +252,7 @@ def readline(prompt = ">> ")
 
     @history.save_line
 
-    @line + "\n"
+    @line
   end
 
   # Reads a line with no prompt