Update builder.rb to fix getting python2 version on Windows

[Ridje]

With single quote on Win 10 IRB returns:

irb(main):007:0> `python -c 'import platform; print(platform.python_version())'`.chomp
File "", line 1
'import
^
SyntaxError: EOL while scanning string literal

So I can't install libv8 via bundle without additional manipulations.
But it works correctly with double quotes:
irb(main):010:0> python -c "import platform; print(platform.python_version())".chomp => "2.7.6"
I suggest use double quotes here, have checked on Mac
python -c "import platform; print(platform.python_version())" works the same way.

[lloeki]

Our efforts are focused on libv8-node and thus libv8 is unmaintained, so I'll close this.

Thanks for the contribution!