Fix environment variable expansion with extras

[jtratner]

Current master tests are failing because of small bug in environment variable expansion.

I.e., this fails:

$ pipenv install 'requests[socks]'
# long traceback ending with
  File "/Users/jtratner/pipenv/pipenv/vendor/pipfile/api.py", line 103, in parse
    parsed_toml = self.inject_environment_variables(toml.loads(content))
  File "/Users/jtratner/pipenv/pipenv/vendor/pipfile/api.py", line 79, in inject_environment_variables
    d[k] = self.inject_environment_variables(v)
  File "/Users/jtratner/pipenv/pipenv/vendor/pipfile/api.py", line 79, in inject_environment_variables
    d[k] = self.inject_environment_variables(v)
  File "/Users/jtratner/pipenv/pipenv/vendor/pipfile/api.py", line 81, in inject_environment_variables
    d[k] = [self.inject_environment_variables(e) for e in v]
  File "/Users/jtratner/pipenv/pipenv/vendor/pipfile/api.py", line 75, in inject_environment_variables
    for k, v in d.items():
AttributeError: 'unicode' object has no attribute 'items'

but this PR gets it to work again - woo!

I will open up an additional PR against the pipfile repo sometime later, just figured it'd be good to unbreak the build :) (I also need to fully grok how extras get converted into a dictionary format)

cc @dvf

[dvf]

Nice catch, thanks @jtratner