Including the online tests under TravisCI

Hello all,

This is potentially contentious: With this commit I have
included the online tests on one of the TravisCI test targets:

https://github.com/biopython/biopython/commit/7b5ab8bd8c09cb725ec798afd2087d25189e2e5b

Plus points: This will catch real mistakes introduced to the master
branch or in pull requests much earlier. It will give more accurate
test coverage reports on codecov.io

https://codecov.io/gh/biopython/biopython

Minus points: This will put some load on the assorted online
servers Biopython tests against (e.g. NCBI BLAST, Entrez,
TogoWS, etc). This will occasionally fail due to temporary
network issues and remote server problems. We may want
to mark the online test as an allowed failure under TravisCI.

I propose to continue the online tests for at least a week, in the
believe they will be useful, but am willing to revert to only doing
the offline tests with TravisCI.

Regards,

Peter.

P.S. Separately, I have been looking at using Tox with TravisCI
with a view to adding other things like pep8 checking
https://github.com/biopython/biopython/issues/840
or building the documentation (in case of adding markup
errors). Work in progress branch here:

https://github.com/peterjc/biopython/tree/tox