Fix create_shared() to be compatible with other shells (e.g. zsh)

Addresses issue #18.

Diagnosis: The command `> log.txt` is intended to create an empty log
named `log.txt`. This command behaves differently for some shells
such as `zsh`. In `zsh`, the command is interpreted as `cat > log.txt`.
(See https://stackoverflow.com/a/15546937 for more details.)

Fix: Use `: > log.txt` to reliably create an empty log.

python/treelite/contrib/util.py

@@ -38,7 +38,7 @@ def _libext():
     return '.so'
 
 def _create_log_cmd_unix(logfile):
-  return ' > {}'.format(logfile)
+  return ': > {}'.format(logfile)
 
 def _save_retcode_cmd_unix(logfile):
   return 'echo $? >> {}'.format(logfile)