fix integration tests and verify download

[DavidHulsman]

• www-us.apache.org was replaced by archive.apache.org to fix the download.
• Added file verification via gpg, as per documentation, so if ./script/hadoop.sh download is run twice (locally) it won't re-download the hadoop-*.*.*.tar.gz file.

Downside: I still get one error when running nosetest (I basically followed .travis.yml to get the tests to run):

======================================================================
ERROR: test_client.TestOptions.test_timeout
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/david/dev/hdfs/test/test_client.py", line 62, in test_timeout
    self.client.status('.')
  File "/home/david/dev/hdfs/hdfs/client.py", line 318, in status
    res = self._get_file_status(hdfs_path, strict=strict)
  File "/home/david/dev/hdfs/hdfs/client.py", line 118, in api_handler
    raise err
hdfs.util.HdfsError: File does not exist: /user/david/.hdfscli
-------------------- >> begin captured logging << --------------------
hdfs.client: INFO: Instantiated <InsecureClient(url='http://0.0.0.0:50070')>.
hdfs.client: INFO: Deleting '' recursively.
hdfs.client: DEBUG: Resolved path '/' to '/'.
urllib3.connectionpool: DEBUG: Starting new HTTP connection (1): 0.0.0.0:50070
urllib3.connectionpool: DEBUG: http://0.0.0.0:50070 "GET /webhdfs/v1/?user.name=david&op=GETHOMEDIRECTORY HTTP/1.1" 200 None
hdfs.client: DEBUG: Updated root to '/user/david/.hdfscli'.
hdfs.client: DEBUG: Resolved path '' to '/user/david/.hdfscli'.
urllib3.connectionpool: DEBUG: http://0.0.0.0:50070 "DELETE /webhdfs/v1/user/david/.hdfscli?user.name=david&recursive=True&op=DELETE HTTP/1.1" 200 None
hdfs.client: INFO: Fetching status for '.'.
hdfs.client: DEBUG: Resolved path '.' to '/user/david/.hdfscli'.
urllib3.connectionpool: DEBUG: http://0.0.0.0:50070 "GET /webhdfs/v1/user/david/.hdfscli?user.name=david&op=GETFILESTATUS HTTP/1.1" 404 None
--------------------- >> end captured logging << ---------------------

----------------------------------------------------------------------
Ran 1 test in 0.510s

FAILED (errors=1)

No clue what that's about, but at least this is one step towards getting travis working again :)

[mtth]

Thanks for the PR @DavidHulsman. The current Travis setup doesn't work anymore so I need to do some additional changes to get this to run again; hoping to merge them together soon.

[mtth]

Merged as part of #179.