Move SPI.begin() per Issue#20

https://github.com/LowPowerLab/SPIFlash/issues/20

SPIFlash.cpp

@@ -68,7 +68,6 @@ void SPIFlash::select() {
   SPI.setDataMode(SPI_MODE0);
   SPI.setBitOrder(MSBFIRST);
   SPI.setClockDivider(SPI_CLOCK_DIV4); //decided to slow down from DIV2 after SPI stalling in some instances, especially visible on mega1284p when RFM69 and FLASH chip both present
-  SPI.begin();
 #endif
   digitalWrite(_slaveSelectPin, LOW);
 }
@@ -96,6 +95,7 @@ boolean SPIFlash::initialize()
   _SPSR = SPSR;
 #endif
   pinMode(_slaveSelectPin, OUTPUT);
+  SPI.begin();
 #ifdef SPI_HAS_TRANSACTION
   _settings = SPISettings(4000000, MSBFIRST, SPI_MODE0);
 #endif