Retry installation commands on Travis CI

As recommended in http://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries/
pyami module: tidy up to meet PEP8 better
Fix run_instances network_interfaces arg doco

network_interfaces takes a
boto.ec2.networkinterface.NetworkInterfaceCollection object, not a list.
Merge pull request #2460 from felixonmars/pyami-tidy

pyami module: tidy up to meet PEP8 better. Fixes #2460.
Merge pull request #2461 from ps-jay/run_instances_network_interfaces_doco

Fix for run_instances() network_interfaces argument documentation. Fixes #2461.
Merge pull request #2457 from danielgtaylor/travis-retry

Retry installation commands on Travis CI. Fixes #2457.