Update ADC test to handle Linux 4.1+ kernel

[pdp7]

@jwcooper I've updated the ADC test to handle Linux 4.1+ kernel.

Tested OK with Linux 4.1.15-ti-rt-r43:

debian@beaglebone:~/adafruit-beaglebone-io-python/test$ uname -a
Linux beaglebone 4.1.15-ti-rt-r43 #1 SMP PREEMPT RT Thu Jan 21 20:13:58 UTC 2016 armv7l GNU/Linux
debian@beaglebone:~/adafruit-beaglebone-io-python/test$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 8.3 (jessie)
Release:    8.3
Codename:   jessie
debian@beaglebone:~/adafruit-beaglebone-io-python/test$ cat /etc/dogtag 
BeagleBoard.org Debian Image 2016-01-24
py ian@beaglebone:~/adafruit-beaglebone-io-python/test$ sudo py.test ./test_adc.p
============================= test session starts ==============================
platform linux2 -- Python 2.7.9, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
rootdir: /home/debian/adafruit-beaglebone-io-python, inifile: 
collected 6 items 

test_adc.py ......

=========================== 6 passed in 3.50 seconds ===========================

Tested OK with Linux 3.8.13-bone79:

debian@beaglebone:~/adafruit-beaglebone-io-python/test$ sudo cat /etc/dogtag 
BeagleBoard.org Debian Image 2015-11-12
debian@beaglebone:~/adafruit-beaglebone-io-python/test$ uname -a
Linux beaglebone 3.8.13-bone79 #1 SMP Tue Oct 13 20:44:55 UTC 2015 armv7l GNU/Linux
debian@beaglebone:~/adafruit-beaglebone-io-python/test$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 7.10 (wheezy)
Release:    7.10
Codename:   wheezy
debian@beaglebone:~/adafruit-beaglebone-io-python/test$ sudo py.test ./test_adc.py
============================= test session starts ==============================
platform linux2 -- Python 2.7.3, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
rootdir: /home/debian/adafruit-beaglebone-io-python, inifile: 
collected 6 items 

test_adc.py ......

========================== 6 passed in 12.07 seconds ===========================