Tweak memory alloc and cleanup

[caternuson]

Tested with Qt PY.

#include "Adafruit_TSL2591.h"

Adafruit_TSL2591 tsl = Adafruit_TSL2591(2591); 

void setup(void) 
{
  Serial.begin(9600);
  while(!Serial);

  Serial.println(F("Starting Adafruit TSL2591 multi begin() Test!"));
}

void loop(void) 
{ 
  if (!tsl.begin()) {
    Serial.println(F("No sensor found ... check your wiring?"));
    while (1);
  } 

  uint16_t x = tsl.getLuminosity(TSL2591_VISIBLE);

  Serial.print(F("[ ")); Serial.print(millis()); Serial.print(F(" ms ] "));
  Serial.print(F("Luminosity: "));
  Serial.println(x, DEC);

  delay(1000);
}