Avoid conflict with Ruby's numbered parameters (#380)

Signed-off-by: Takahiro SATOH <zalt50cc@gmail.com>

Author: zalt50

lib/iruby/backend.rb

@@ -18,7 +18,7 @@ def eval(code, store_history)
       # TODO Add IRuby.cache_size which controls the size of the Out array
       # and sets the oldest entries and _<n> variables to nil.
       if store_history
-        b.local_variable_set("_#{Out.size}", out)
+        b.local_variable_set("_o#{Out.size}", out)
         b.local_variable_set("_i#{In.size}", code)
 
         Out << out