Updated for recent versions of PyMongo & add travis CI tests

[rupello]

I updated to work with recent library versions

Tests are passing for Travis CI for the following:

python:

• "2.7"
• "3.4"
• "3.5"
• "3.6"
• "3.7-dev"

env:

• PYMONGO_VERSION=3.7
• PYMONGO_VERSION=3.0
• PYMONGO_VERSION=2.4