Update node version in travis tests

0.6.0

Merge pull request #65 from balderdashy/upgrade-mime

lookup() renamed to getType() in mime 2

Update version of mime because of vulnerability

0.6.0-8

Bump aws-sdk dependency to attempt to solve https://github.com/aws/aws-sdk-js/issues/1761#issuecomment-455723620

Merge pull request #64 from balderdashy/experimental-revamp

Remove node 4 from .travis.yml to get around unsupported syntax in mocha.

Stub benchmarks

0.6.0-7

Use .skipperFd if provided, and only use .fd as a worst-case-scenario fallback.

More details in assertion error msg

0.6.0-6

Add assertion to make it easier to debug when userland code doesn't provide the expected fd property on incoming file streams.