Make virtualenv.vim compatible with Python 3.

The `virtualenv.vim` file used the `execfile()` function to run the `activate_this.py` script from the active virtual environment. However, `execfile()` was removed in Python 3. This replaces the call to `execfile()` with a process that should be compatible with Python 2 and Python 3:

1. Read the script contents into a string.
2. Pass the string to `compile()` to get a compiled script object.
3. Pass the compiled script object to `exec()`.