configure: prevent ANSI escape codes in stdout of Python version check

Fixes Python version checking with some shells/terminals outputting ANSI escape codes by adding execFile() options `{env:{'TERM':'dumb'}}`:

Before:
> gyp verb check python version 'python -c 
> "import platform; print(platform.python_version());" returned: **"\u001b[?1034h2.7.2\n"**
> gyp ERR! configure error 
> gyp ERR! stack Error: Python executable "python" is v2.7.2, which is not supported by gyp. 
> gyp ERR! stack You can pass the --python switch to point to Python >= v2.5.0 & < 3.0.0.

After:
> gyp verb check python version 'python -c 
> "import platform; print(platform.python_version());"' returned: **"2.7.2\n"**