Minor updates to BridgeDB's dependencies

Some minor errors occurred when trying to setup BridgeDB on a fresh system:

The link to the get-pip script is outdated and no longer available. The new address would be: [https://bootstrap.pypa.io/get-pip.py]

When setup.py is executed the following error occurs: error: The 'pyasn1' distribution was not found and is required by service-identity. Therefore i suggest adding pyasn1 to requirements.txt.

Since BridgeDB's port seems as good as done, this might not be necessary but the pylint version 2.4.4 in .test.requirements.txt is only supporting Python 3. For the installation with Python 2.7, pylint version 1.9.5 would be required.