Remove httplib2

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Update: I don't see httplib2 anywhere in KC's code except for requirements files (checked in https://github.com/kobotoolbox/kobocat/tree/kobokitten-py3-django2-new-settings) —jnm

Original description:

As identified in #158, the currently-installed version of httplib2 results in certificate verification errors such as the following on apparently-acceptable sites like www.openfn.org:

SSLHandshakeError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)

Without upgrading httplib2 or resorting to using disable_ssl_certificate_validation=True, we can easily substitute in the already-installed requests library, which is not subject to these verification errors, and drop a dependency in the process.