Removed extra error from ruby on incorrect exiting

When phantomjs process was interrupted (i.e. ith ^C) it's exitstatus is
nil, so `exit` function prints useless error message about nil to int
conversion.

Now explicitly exit with 1, which is a default ruby exit behaviour and
stays in sync with a python bin script.

@@ -59,4 +59,4 @@ if system(CASPER_COMMAND.join(" ")).nil?
     puts "Fatal: Did you install phantomjs?"
 end
 
-exit $?.exitstatus
+exit $?.exitstatus || 1