tests for rest.py fail in Travis because of API rate limiting

[boegel]

ERROR: Test all request methods
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Traceback (most recent call last):
  File "/home/travis/build/hpcugent/vsc-base/test/rest.py", line 74, in test_request_methods
    status, body = self.client.head()
  File "lib/vsc/utils/rest.py", line 128, in head
    return self.request(self.HEAD, url, None, headers)
  File "lib/vsc/utils/rest.py", line 175, in request
    conn = self.get_connection(method, url, body, headers)
  File "lib/vsc/utils/rest.py", line 206, in get_connection
    connection = self.opener.open(request)
  File "/opt/python/2.6.9/lib/python2.6/urllib2.py", line 397, in open
    response = meth(req, response)
  File "/opt/python/2.6.9/lib/python2.6/urllib2.py", line 510, in http_response
    'http', request, response, code, msg, hdrs)
  File "/opt/python/2.6.9/lib/python2.6/urllib2.py", line 435, in error
    return self._call_chain(*args)
  File "/opt/python/2.6.9/lib/python2.6/urllib2.py", line 369, in _call_chain
    result = func(*args)
  File "/opt/python/2.6.9/lib/python2.6/urllib2.py", line 518, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
HTTPError: HTTP Error 403: Forbidden

We can fix this by using a GitHub token in Travis, and skip the tests that require a token when the token is missing (token will never be available when testing PRs because of security reasons).