Bump pymongo from 2.4.1 to 2.5.2 in /examples/pymongo

Bumps [pymongo](https://github.com/mongodb/mongo-python-driver) from 2.4.1 to 2.5.2.
- [Release notes](https://github.com/mongodb/mongo-python-driver/releases)
- [Changelog](https://github.com/mongodb/mongo-python-driver/blob/master/doc/changelog.rst)
- [Commits](mongodb/mongo-python-driver@2.4.1...2.5.2)

---
updated-dependencies:
- dependency-name: pymongo
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

examples/pymongo/requirements.txt

@@ -1,4 +1,4 @@
 Flask
 Flask-Admin
-pymongo==2.4.1
+pymongo==2.5.2
 bson