0.2b2 doesn't install on python 2.6

When I do a

python setup.py --use-system-libraries --enable-legacy

on a 2.6 system, the installation fails with
```
  File "/tmp/buildd/python-astropy-0.2~b2/astropy/setup_helpers.py", line 1247, in pkg_config
    output = subprocess.check_output(command, shell=True)
```

The problem is similar to issue #42 that under 2.6, subprocess.check_output does still not exist.

Similarly, it could be fixed with
```diff
--- a/astropy/setup_helpers.py
+++ b/astropy/setup_helpers.py
@@ -1244,7 +1244,8 @@
     command = "pkg-config --libs --cflags {0}".format(' '.join(packages)),
 
     try:
-        output = subprocess.check_output(command, shell=True)
+        process = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE)
+        output = process.communicate()[0].strip()
     except subprocess.CalledProcessError as e:
         lines = [
             "pkg-config failed.  This may cause the build to fail below.",
```