Make VIRTUAL_ENV actually importable

I had this strange error when I run ``py.test`` instead of ``tox``:

```
name = 'site-packages.numpy.core.umath'
path = '/home/takafumi/.virtualenvs/mypy/lib/python2.7'

    def load_module(name, path):
        if path:
            self.sys_path.insert(0, path)
    
        temp, sys.path = sys.path, self.sys_path
        content = {}
        try:
>           exec_function('import %s as module' % name, content)

jedi/builtin.py:90: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

source = 'import site-packages.numpy.core.umath as module'
global_map = {'__builtins__': {'ArithmeticError': <type 'exceptions.ArithmeticError'>, 'AssertionError': <type 'exceptions.AssertionError'>, 'AttributeError': <type 'exceptions.AttributeError'>, 'BaseException': <type 'exceptions.BaseException'>, ...}}

>   ???
E     File "<string>", line 1
E       import site-packages.numpy.core.umath as module
E                  ^
E   SyntaxError: invalid syntax

blub:2: SyntaxError
================= 1 failed, 40 passed, 1 skipped in 6.88 seconds =================
py.test test/test_regression.py  7.26s user 0.18s system 95% cpu 7.802 total
```

It turned out that this is because `sys.path` is used in `BuiltinModule.module` when searching for module.  As we are using real import machinery for builtin module, I think we should just add it to `sys.path` rather than Jedi's internal copy.