Update setup.py to always require pillow and not import PIL

It seems that in older versions of setuptools (or maybe even easy_install) that importing certain libraries in setup.py causes an infinite loop and eats up memory until it gets killed by the kernel.

setup.py

@@ -40,7 +40,7 @@
 BASE_PATH = os.path.sep.join(os.path.dirname(
     os.path.realpath(__file__)).split(os.path.sep))
 
-requires = ['numpy >=1.4.1', 'pyresample', 'trollsift', 'trollimage', 'pykdtree', 'six']
+requires = ['numpy >=1.4.1', 'pillow', 'pyresample', 'trollsift', 'trollimage', 'pykdtree', 'six']
 
 if sys.version < '2.7':
     requires.append('ordereddict')
@@ -51,12 +51,6 @@
 if sys.version < '3.0':
     test_requires.append('mock')
 
-try:
-    from PIL import Image
-except ImportError:
-    requires.append("pillow")
-
-
 def _config_data_files(base_dirs, extensions=(".cfg",)):
     """Find all subdirectory configuration files