Fix python 2.6 errors

[tony]

This issue occurs with files on python 2.6 and leads to failed builds on travis.

test_freeze_config (tmuxp.testsuite.test_workspacefreezer.FreezeTest) ... ok
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ERROR: test_export_json (tmuxp.testsuite.test_config.ImportExportTest)
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Traceback (most recent call last):
  File "/home/travis/build/tony/tmuxp/tmuxp/testsuite/test_config.py", line 71, in test_export_json
    buf = open(json_config_file, 'w')
IOError: [Errno 2] No such file or directory: '/home/travis/build/tony/tmuxp/tmuxp/testsuite/.tmuxp/config.json'
======================================================================
ERROR: test_export_yaml (tmuxp.testsuite.test_config.ImportExportTest)
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Traceback (most recent call last):
  File "/home/travis/build/tony/tmuxp/tmuxp/testsuite/test_config.py", line 89, in test_export_yaml
    buf = open(yaml_config_file, 'w')
IOError: [Errno 2] No such file or directory: '/home/travis/build/tony/tmuxp/tmuxp/testsuite/.tmuxp/config.yaml'
======================================================================
ERROR: test_scan_config (tmuxp.testsuite.test_config.ImportExportTest)
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Traceback (most recent call last):
  File "/home/travis/build/tony/tmuxp/tmuxp/testsuite/test_config.py", line 101, in test_scan_config
    buf = open(garbage_file, 'w')
IOError: [Errno 2] No such file or directory: '/home/travis/build/tony/tmuxp/tmuxp/testsuite/.tmuxp/config.psd'
----------------------------------------------------------------------
Ran 72 tests in 34.752s
FAILED (errors=3)
The command "python setup.py test" exited with 1.
Done. Your build exited with 1.