Add a ci-requirements.txt for dependencies needed during CI.

We recently removed `mysqlclient` as a dependency in
`dev-requirements.txt`, in order to simplify getting a development tree
going. We only needed that dependency for some unit tests.

This change introduces a `ci-requirements.txt` file, which will be used
for any dependencies needed in a CI setup. It currently lists only the
`mysqlclient` dependency we removed. The Travis CI configuration has
been updated to use it.

Testing Done:
Tested that `pip install`ing from the file worked.

Reviewed at https://reviews.reviewboard.org/r/10845/

.travis.yml

@@ -20,6 +20,7 @@ install:
       fi
     - python setup.py develop
     - pip install -r dev-requirements.txt
+    - pip install -r ci-requirements.txt
 
 script:
     # Set up the origin/master branch, for doc building.

ci-requirements.txt

@@ -0,0 +1 @@
+mysqlclient