Prevent duplicate require calls

This bit of code doesn't check for duplicates

    while( match = reg.exec( nunjucksCompiledStr ) ) {
        var templateRef = match[1];
        compiledTemplate += 'require( "' + templateRef + '" );\n';
    }

Should be easy to fix, I'm just writing it down to not forget later. Don't have much time at the moment, but will open a pr if it is welcome :)