Add -Wno-strict-aliasing to CFLAGS to ignore gcc warning

GitHub issue #133 was raised by @archey:
Compilation issues due to strict aliasing

These errors occured when building with gcc 6.3:
```
gcc -pthread -DNDEBUG -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -march=x86-64 -mtune=generic -O3 -pipe -fstack-protector-all --param=ssp-buffer-size=4 -D_FORTIFY_SOURCE=2 -fPIC -DBBBVERSION41 -I/usr/include/python2.7 -c source/py_gpio.c -o build/temp.linux-x86_64-2.7/source/py_gpio.o -Wall -Werror -Wextra -Wno-missing-field-initializers
source/py_gpio.c: In function ‘py_event_detected’:
source/py_gpio.c:433:7: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
       Py_RETURN_TRUE;
       ^~~~~~~~~~~~~~
source/py_gpio.c:435:7: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
       Py_RETURN_FALSE;
       ^~~~~~~~~~~~~~~
source/py_gpio.c: In function ‘py_wait_for_edge’:
source/py_gpio.c:479:7: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
       Py_RETURN_FALSE;
       ^~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
error: command 'gcc' failed with exit status 1
```

Py_RETURN_TRUE and Py_RETURN_FALSE are part of the Python C API:
https://docs.python.org/2/c-api/bool.html

Author: pdp7

setup.py

@@ -21,7 +21,7 @@
 else:
     kernel41 = None
 
-CFLAGS = ['-Wall', '-Werror', '-Wextra', '-Wno-missing-field-initializers' ]
+CFLAGS = ['-Wall', '-Werror', '-Wextra', '-Wno-missing-field-initializers', '-Wno-strict-aliasing' ]
 
 classifiers = ['Development Status :: 3 - Alpha',
                'Operating System :: POSIX :: Linux',