Pip build test CI failures
I've seen a couple of these. Is this intermittent? Any idea what's going on?

https://travis-ci.org/pandas-dev/pandas/jobs/340640051

```
    running build_ext
    pandas._libs.algos: -> [['pandas/_libs/algos.c']]
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-nv04tz46-build/setup.py", line 741, in <module>
        **setuptools_kwargs)
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/setuptools/__init__.py", line 129, in setup
        return distutils.core.setup(**attrs)
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/core.py", line 148, in setup
        dist.run_commands()
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/dist.py", line 955, in run_commands
        self.run_command(cmd)
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/dist.py", line 974, in run_command
        cmd_obj.run()
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/setuptools/command/install.py", line 61, in run
        return orig.install.run(self)
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/command/install.py", line 545, in run
        self.run_command('build')
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/cmd.py", line 313, in run_command
        self.distribution.run_command(command)
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/dist.py", line 974, in run_command
        cmd_obj.run()
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/command/build.py", line 135, in run
        self.run_command(cmd_name)
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/cmd.py", line 313, in run_command
        self.distribution.run_command(command)
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/dist.py", line 974, in run_command
        cmd_obj.run()
      File "/home/travis/miniconda3/envs/pandas/lib/python3.6/distutils/command/build_ext.py", line 339, in run
        self.build_extensions()
      File "/tmp/pip-nv04tz46-build/setup.py", line 367, in build_extensions
        self.check_cython_extensions(self.extensions)
      File "/tmp/pip-nv04tz46-build/setup.py", line 364, in check_cython_extensions
        """.format(src=src))
    Exception: Cython-generated file 'pandas/_libs/algos.c' not found.
                    Cython is required to compile pandas from a development branch.
                    Please install Cython or download a release package of pandas.
    
```