Use Pillow if PIL is not found.

Try to fix the travis build error by replacing the PIL dependency.  And remove
the system pyexiv2 for travis as it is not used in the virtualenv.

.travis.yml

@@ -5,7 +5,7 @@ python:
 before_install:
     # Dependencies to build PIL
     - sudo apt-get update -qq
-    - sudo apt-get install -qq python-pyexiv2 libfreetype6-dev libjpeg8-dev zlib1g-dev
+    - sudo apt-get install -qq libfreetype6-dev libjpeg8-dev zlib1g-dev
 install:
     - pip install nose unittest2 --use-mirrors
     - pip install . --use-mirrors

setup.py

@@ -3,10 +3,15 @@
 import sys
 from setuptools import setup, find_packages
 
-requires = ['PIL', 'jinja2', 'Markdown', 'clint']
+requires = ['jinja2', 'Markdown', 'clint']
 if sys.version_info < (2, 7):
     requires.append('argparse')
 
+try:
+    from PIL import Image, ImageOps  # NOQA
+except ImportError:
+    requires += ['Pillow']
+
 entry_points = {
     'console_scripts': ['sigal = sigal:main']
 }