Constrain to older pip that might be less broken

Trying to fix appveyor failure:

```
Obtaining file:///C:/projects/tahoe-lafs
    Complete output from command python setup.py egg_info:
    warning: no previously-included files found matching 'pyproject.toml'

    Installed c:\projects\tahoe-lafs\.eggs\setuptools-41.0.1-py2.7.egg
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "C:\projects\tahoe-lafs\setup.py", line 297, in <module>
        **setup_args
      File "c:\python27\Lib\distutils\core.py", line 111, in setup
        _setup_distribution = dist = klass(attrs)
      File "C:\projects\tahoe-lafs\.tox\coverage\lib\site-packages\setuptools\dist.py", line 272, in __init__
        _Distribution.__init__(self,attrs)
      File "c:\python27\Lib\distutils\dist.py", line 287, in __init__
        self.finalize_options()
      File "C:\projects\tahoe-lafs\.tox\coverage\lib\site-packages\setuptools\dist.py", line 327, in finalize_options
        ep.load()(self, ep.name, value)
      File "C:\projects\tahoe-lafs\.tox\coverage\lib\site-packages\pkg_resources\__init__.py", line 2202, in load
        return self.resolve()
      File "C:\projects\tahoe-lafs\.tox\coverage\lib\site-packages\pkg_resources\__init__.py", line 2212, in resolve
        raise ImportError(str(exc))
    ImportError: 'module' object has no attribute 'check_specifier'
```

.appveyor.yml

@@ -13,7 +13,7 @@ environment:
 
 install:
   - |
-    %PYTHON%\python.exe -m pip install -U pip
+    %PYTHON%\python.exe -m pip install -U 'pip<19'
     %PYTHON%\python.exe -m pip install wheel tox virtualenv
 
 # note: