.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` fails with `StopIteration` and the below stack
trace. I don't know if this is a bug with astroid 1.2.0, or if 1.2.0
isn't supposed to work with pylint 1.1.0 (pylint is up to 1.3.0 now,
but on travis we have pylint locked to 1.1.0 so that it gives the same
results as for developers running Ubuntu 14.04).

    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