Moved last_update_times outside main() for unit tests

The unit tests fail since last_update_times is defined initially in main()

```
running build_ext
test_Glances_getCore (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getCpu (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getDiskIO (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getFs (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getHDDTemp (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getMem (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getMemSwap (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getNetwork (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getPerCpu (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getProcess (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getSensors (glances.tests.test_glances.TestGlancesStat) ... ERROR
test_Glances_getSystem (glances.tests.test_glances.TestGlancesStat) ... ERROR

======================================================================
ERROR: test_Glances_getCore (glances.tests.test_glances.TestGlancesStat)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/will/Projects/glances/glances/tests/test_glances.py", line 35, in setUp
    self.stats.update()
  File "/home/will/Projects/glances/glances/glances.py", line 1236, in update
    self.__update__(input_stats)
  File "/home/will/Projects/glances/glances/glances.py", line 1146, in __update__
    time_since_update = getTimeSinceLastUpdate('net')
  File "/home/will/Projects/glances/glances/glances.py", line 4033, in getTimeSinceLastUpdate
    last_time = last_update_times.get(IOType)
NameError: global name 'last_update_times' is not defined

======================================================================
ERROR: test_Glances_getCpu (glances.tests.test_glances.TestGlancesStat)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/will/Projects/glances/glances/tests/test_glances.py", line 35, in setUp
    self.stats.update()
  File "/home/will/Projects/glances/glances/glances.py", line 1236, in update
    self.__update__(input_stats)
  File "/home/will/Projects/glances/glances/glances.py", line 1146, in __update__
    time_since_update = getTimeSinceLastUpdate('net')
  File "/home/will/Projects/glances/glances/glances.py", line 4033, in getTimeSinceLastUpdate
    last_time = last_update_times.get(IOType)
NameError: global name 'last_update_times' is not defined

======================================================================
ERROR: test_Glances_getDiskIO (glances.tests.test_glances.TestGlancesStat)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/will/Projects/glances/glances/tests/test_glances.py", line 35, in setUp
    self.stats.update()
  File "/home/will/Projects/glances/glances/glances.py", line 1236, in update
    self.__update__(input_stats)
  File "/home/will/Projects/glances/glances/glances.py", line 1146, in __update__
    time_since_update = getTimeSinceLastUpdate('net')
  File "/home/will/Projects/glances/glances/glances.py", line 4033, in getTimeSinceLastUpdate
    last_time = last_update_times.get(IOType)
NameError: global name 'last_update_times' is not defined

======================================================================
ERROR: test_Glances_getFs (glances.tests.test_glances.TestGlancesStat)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/will/Projects/glances/glances/tests/test_glances.py", line 35, in setUp
    self.stats.update()
  File "/home/will/Projects/glances/glances/glances.py", line 1236, in update
    self.__update__(input_stats)
  File "/home/will/Projects/glances/glances/glances.py", line 1146, in __update__
    time_since_update = getTimeSinceLastUpdate('net')
  File "/home/will/Projects/glances/glances/glances.py", line 4033, in getTimeSinceLastUpdate
    last_time = last_update_times.get(IOType)
NameError: global name 'last_update_times' is not defined

======================================================================
ERROR: test_Glances_getHDDTemp (glances.tests.test_glances.TestGlancesStat)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/will/Projects/glances/glances/tests/test_glances.py", line 35, in setUp
    self.stats.update()
  File "/home/will/Projects/glances/glances/glances.py", line 1236, in update
    self.__update__(input_stats)
  File "/home/will/Projects/glances/glances/glances.py", line 1146, in __update__
    time_since_update = getTimeSinceLastUpdate('net')
  File "/home/will/Projects/glances/glances/glances.py", line 4033, in getTimeSinceLastUpdate
    last_time = last_update_times.get(IOType)
NameError: global name 'last_update_times' is not defined
...
```

I define and initialize last_update_times outside of main(), and now unit tests pass.