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()`.
Merge pull request #293 from mlmoses/fix-virtualenv-execfile

Make virtualenv.vim compatible with Python 3.