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EDITED

I have tried to change the serial port by using the class definition below, but it didn't recognize Serial1.

class NewHardware : public ArduinoHardware
{
   public:
   NewHardware():ArduinoHardware(&Serial1, 57600){};
};

ros::NodeHandle_<NewHardware>  nh;

So, I added this line in the beginning of my sketch

# define USBCON

which would change the serial port, according to some page on the internet. And now, I get a new error message.

[INFO] [WallTime: 1470535945.212095] ROS Serial Python Node
[INFO] [WallTime: 1470535945.215136] Connecting to /dev/rfcomm0 at 57600 baud
Traceback (most recent call last):
  File "/opt/ros/indigo/lib/rosserial_python/serial_node.py", line 80, in <module>
    client = SerialClient(port_name, baud)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rosserial_python/SerialClient.py", line 385, in __init__
    self.requestTopics()
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rosserial_python/SerialClient.py", line 392, in requestTopics
    self.port.flushInput()
  File "/usr/lib/python2.7/dist-packages/serial/serialposix.py", line 500, in flushInput
    termios.tcflush(self.fd, TERMIOS.TCIFLUSH)
termios.error: (5, 'Input/output error')

Any ideas? This new error message doesn't even say what the problem is.

"SOLVED"

Well, I ended up choosing another way to accomplish my goal. I've developed (not alone) a ROS code, on my laptop, that gets data directly from serial port. This is the code:

#include "ros/ros.h"
#include <fcntl.h>
#include <termios.h>
#include <errno.h>
#include <sys/ioctl.h>

int main(int argc, char **argv)
{
    ros::init (argc,argv,"imu_node");
    ros::NodeHandle nh;
    ros::Rate loop_rate(50); //Hz
    std::string aux;

    //Begin serial communication with Arduino
        struct termios toptions;
    int fd, n;
    float roll, pitch, yaw;

    nh.getParam("imu_node/serial_port",aux);
    ROS_INFO_STREAM(aux);

    fd = open(aux.c_str(), O_RDWR | O_NOCTTY);

    /* wait for the Arduino to reboot */
    usleep(3500000);

    /* get current serial port settings */
    tcgetattr(fd, &toptions);
    /* set 9600 baud both ways */
    cfsetispeed(&toptions, B115200);
    cfsetospeed(&toptions, B115200);
    /* 8 bits, no parity, no stop bits */
    toptions.c_cflag &= ~PARENB;
    toptions.c_cflag &= ~CSTOPB;
    toptions.c_cflag &= ~CSIZE;
    toptions.c_cflag |= CS8;
    /* Canonical mode */
    toptions.c_lflag |= ICANON;
    /* commit the serial port settings */
    tcsetattr(fd, TCSANOW, &toptions);

    while(ros::ok())
    {
        char buf[64]="temp text";
        write(fd, "I\n", 2);
        usleep(500);
        /* Receive string from Arduino */
        do
        {
            n = read(fd, buf, 32);
        }while (n < 10);
        /* insert terminating zero in the string */
        buf[n] = 0;     
        sscanf(buf, "I|%f|%f|%f|*\r\n", &yaw, &pitch, &roll);
        ROS_INFO_STREAM(buf);
            ROS_INFO_STREAM("accel_x: " << roll);
            ROS_INFO_STREAM("accel_y: " << pitch);
            ROS_INFO_STREAM("yaw: " << yaw);

        ros::spinOnce();
        loop_rate.sleep();
    }
}

Then, I link the rfcomm0 port created by the bluetooth connection to another random serial port, because, apparently, the parameter setting doesn't work with the original bluetooth port (rfcomm0).

$ sudo ln -s /dev/rfcomm0 /dev/ttyBT

And finally, I set the node parameter and run the node.

$ rosparam set /imu_node/serial_port /dev/ttyBT
$ rosrun imu imu