.travis.yml: Lock astroid to version 1.1.0

astroid is a library used by pylint. Since astroid 1.2.0 was released,
our `make check-pylint` on Travis fails with `StopIteration` and the
below stack trace while checking tests/test_ocr.py. See bug #42 in the
astroid bug tracker[1].

    File "/usr/local/bin/pylint", line 9, in <module>
      load_entry_point('pylint==1.1.0', 'console_scripts', 'pylint')()
    File "/usr/local/lib/python2.7/dist-packages/pylint/__init__.py", line 21, in run_pylint
      Run(sys.argv[1:])
    File "/usr/local/lib/python2.7/dist-packages/pylint/lint.py", line 1040, in __init__
      linter.check(args)
    File "/usr/local/lib/python2.7/dist-packages/pylint/lint.py", line 626, in check
      self.check_astroid_module(astroid, walker, rawcheckers, tokencheckers)
    File "/usr/local/lib/python2.7/dist-packages/pylint/lint.py", line 712, in check_astroid_module
      walker.walk(astroid)
    File "/usr/local/lib/python2.7/dist-packages/pylint/utils.py", line 697, in walk
      self.walk(child)
    File "/usr/local/lib/python2.7/dist-packages/pylint/utils.py", line 694, in walk
      cb(astroid)
    File "/usr/local/lib/python2.7/dist-packages/pylint/checkers/base.py", line 246, in visit_function
      self._check_redefinition(node.is_method() and 'method' or 'function', node)
    File "/usr/local/lib/python2.7/dist-packages/astroid/scoped_nodes.py", line 670, in is_method
      return self.type != 'function' and isinstance(self.parent.frame(), Class)
    File "/usr/local/lib/python2.7/dist-packages/logilab/common/decorators.py", line 155, in __get__
      val = self.wrapped(inst)
    File "/usr/local/lib/python2.7/dist-packages/astroid/scoped_nodes.py", line 536, in _function_type
      _type = _infer_decorator_callchain(node)
    File "/usr/local/lib/python2.7/dist-packages/astroid/scoped_nodes.py", line 501, in _infer_decorator_callchain
      result = current.infer_call_result(current.parent).next()
    StopIteration

[1]: https://bitbucket.org/logilab/astroid/issue/42/pylint-with-astroid-120-fails-with