Stop configuring install_command in tox.

Currently, we are overriding 'install_command' to use 'pip'. This is
considered poor behavior and 'python -m pip' should be used instead:

https://snarky.ca/why-you-should-use-python-m-pip/

It turns out that this is the the default value provided by tox:

https://tox.readthedocs.io/en/latest/config.html#conf-install_command

So we can remove the line and simply use the default value.

Change-Id: Ibcdd3a9f7e7ea421e906cf1d96f95b516008b5d7

tox.ini

@@ -4,7 +4,6 @@ distribute = False
 envlist = py27,py37,pep8
 
 [testenv]
-install_command = pip install {opts} {packages}
 whitelist_externals =
   find
 setenv =