avoid pickle assertion failure on something in boto

I started playing with elasticluster this morning, and experienced an assertion failure when it tried to write the pickled cluster state. It seems to fail on something in the boto instances, and it seems reasonable (and backwards compatible, since the pickle format ID id stored in the data stream) to use the highest protocol available, which avoids the assertion failure.


```
Starting cluster `aws-slurm` with 2 compute nodes.
Starting cluster `aws-slurm` with 1 frontend nodes.
(this may take a while...)
Traceback (most recent call last):
  File "/home/jwm/.bin/env/elasticluster/bin/elasticluster", line 9, in <module>
    load_entry_point('elasticluster==1.1-dev', 'console_scripts', 'elasticluster')()
  File "/home/jwm/.bin/env/elasticluster/local/lib/python2.7/site-packages/elasticluster-1.1_dev-py2.7.egg/elasticluster/main.py", line 175, in main
    app.run()
  File "/home/jwm/.bin/env/elasticluster/local/lib/python2.7/site-packages/pyCLI-2.0.3-py2.7.egg/cli/app.py", line 245, in run
    return self.post_run(returned)
  File "/home/jwm/.bin/env/elasticluster/local/lib/python2.7/site-packages/pyCLI-2.0.3-py2.7.egg/cli/app.py", line 241, in run
    returned = self.main(*args)
  File "/home/jwm/.bin/env/elasticluster/local/lib/python2.7/site-packages/elasticluster-1.1_dev-py2.7.egg/elasticluster/main.py", line 160, in main
    return self.params.func()
  File "/home/jwm/.bin/env/elasticluster/local/lib/python2.7/site-packages/elasticluster-1.1_dev-py2.7.egg/elasticluster/subcommands.py", line 70, in __call__
    return self.execute()
  File "/home/jwm/.bin/env/elasticluster/local/lib/python2.7/site-packages/elasticluster-1.1_dev-py2.7.egg/elasticluster/subcommands.py", line 187, in execute
    cluster.start(min_nodes=min_nodes)
  File "/home/jwm/.bin/env/elasticluster/local/lib/python2.7/site-packages/elasticluster-1.1_dev-py2.7.egg/elasticluster/cluster.py", line 334, in start
    self.repository.save_or_update(self)
  File "/home/jwm/.bin/env/elasticluster/local/lib/python2.7/site-packages/elasticluster-1.1_dev-py2.7.egg/elasticluster/repository.py", line 197, in save_or_update
    pickle.dump(cluster, storage)
  File "/usr/lib/python2.7/pickle.py", line 1370, in dump
    save(v)
  File "/usr/lib/python2.7/pickle.py", line 331, in save
    self.save_reduce(obj=obj, *rv)
  File "/usr/lib/python2.7/pickle.py", line 419, in save_reduce
    save(state)
  File "/usr/lib/python2.7/pickle.py", line 286, in save
    f(self, obj) # Call unbound method with explicit self
  File "/usr/lib/python2.7/pickle.py", line 649, in save_dict
    self._batch_setitems(obj.iteritems())
  File "/usr/lib/python2.7/pickle.py", line 663, in _batch_setitems
    save(v)
  File "/usr/lib/python2.7/pickle.py", line 286, in save
    f(self, obj) # Call unbound method with explicit self
  File "/usr/lib/python2.7/pickle.py", line 649, in save_dict
    self._batch_setitems(obj.iteritems())
  File "/usr/lib/python2.7/pickle.py", line 663, in _batch_setitems
    save(v)
  File "/usr/lib/python2.7/pickle.py", line 331, in save
    self.save_reduce(obj=obj, *rv)
  File "/usr/lib/python2.7/pickle.py", line 419, in save_reduce
    save(state)
  File "/usr/lib/python2.7/pickle.py", line 286, in save
    f(self, obj) # Call unbound method with explicit self
  File "/usr/lib/python2.7/pickle.py", line 649, in save_dict
    self._batch_setitems(obj.iteritems())
  File "/usr/lib/python2.7/pickle.py", line 663, in _batch_setitems
    save(v)
  File "/usr/lib/python2.7/pickle.py", line 331, in save
    self.save_reduce(obj=obj, *rv)
  File "/usr/lib/python2.7/pickle.py", line 405, in save_reduce
    self.memoize(obj)
  File "/usr/lib/python2.7/pickle.py", line 244, in memoize
    assert id(obj) not in self.memo
AssertionError
```