Add python 3.6 to travis python versions.  Build is currently failing for Python 3.6 with the error 'Error creating virtualenv ...', see https://travis-ci.org/approvals/ApprovalTests.Python/builds/412805547.
Add python 3.6 to travis python versions.  Build is currently failing for Python 3.6 with the error 'Error creating virtualenv ...', see https://travis-ci.org/approvals/ApprovalTests.Python/builds/412805547.

approvals/ApprovalTests.Python,tdpreece/ApprovalTests.Python,approvals/ApprovalTests.Python,approvals/ApprovalTests.Python