fix(docs): Install the client using pip

The previous install method was failing on my mac (python installed using homebrew) with the following error.

```
Traceback (most recent call last):
  File "/usr/local/bin/deis", line 8, in <module>
    load_entry_point('deis==0.10.0', 'console_scripts', 'deis')()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 318, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 2221, in load_entry_point
    return ep.load()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 1954, in load
    entry = __import__(self.module_name, globals(),globals(), ['__name__'])
  File "/Users/johannes/Documents/Web/deis/client/deis.py", line 8, in <module>
    load_entry_point('deis==0.10.0', 'console_scripts', 'deis')()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 318, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 2221, in load_entry_point
    return ep.load()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 1959, in load
    raise ImportError("%r has no %r attribute" % (entry,attr))
ImportError: <module 'deis' from '/Users/johannes/Documents/Web/deis/client/deis.py'> has no 'main' attribute
```