Update gyro calibration implementation

*dmp_enable_feature* (**DMP_FEATURE_GYRO_CAL**) needs to be called in conjunction with **DMP_FEATURE_SEND_CAL_GYRO**
See: https://github.com/sparkfun/9DOF_Razor_IMU/blob/master/Libraries/Arduino/src/util/inv_mpu_dmp_motion_driver.c#L1090

Brought up by in this issue: #10
Tested to work.

Firmware/_9DoF_Razor_M0_Firmware/_9DoF_Razor_M0_Firmware.ino

@@ -323,6 +323,7 @@ bool initIMU(void)
   {
     // Gyro calibration re-calibrates the gyro after a set amount
     // of no motion detected
+    dmpFeatureMask |= DMP_FEATURE_GYRO_CAL;
     dmpFeatureMask |= DMP_FEATURE_SEND_CAL_GYRO;
   }
   else