Abandon all hope of fixing installation on Py 2.6

Paramiko depends on `PyNaCl` (for cryptography), which in turn depends
on `pycparser`, which dropped support for Py 2.6 in version 2.19 (but
version 2.18 is still fine). However, `PyNaCl` seems to vendorize
`pycparser` or at least get its own private copy of it, as all my
attempts to pin down `pycparser` to version 2.18 failed...

setup.py

@@ -74,11 +74,6 @@ def run_tests(self):
         # - PyCLI requires argparse,
         'argparse',
         'lockfile==0.11.0',
-        # Paramiko depends on PyNaCl which depends on `pycparser`,
-        # which dropped support for Py2.6 in 2.19 but we need to pin
-        # `PyNaCl` itself: attempts to pin `pycparser` result in
-        # failure of installation attempts on Travis CI...
-        'PyNaCl==1.0.1',
         # Paramiko ceased support for Python 2.6 in version 2.4.0
         'paramiko<2.4', 'pycrypto',
         # parsedatetime officially dropped supprt for Py 2.6 in version 1.0