Depend on the cffi package

Let both pip and Travis know what we need.

.travis.yml

@@ -11,6 +11,7 @@ env: LIBGIT2=~/libgit2/_install/ LD_LIBRARY_PATH=~/libgit2/_install/lib
 
 before_install:
   - sudo apt-get install cmake
+  - pip install cffi
   - "./.travis.sh"
 
 script:

setup.py

@@ -185,6 +185,7 @@ setup(name='pygit2',
       long_description=long_description,
       packages=['pygit2'],
       package_data={'pygit2': ['decl.h']},
+      install_requires=['cffi'],
       ext_modules=[
           Extension('_pygit2', pygit2_exts,
                     include_dirs=[libgit2_include, 'include'],