Remove proxy-agent, not used (#219)

proxy-agent was made redundant after #172, which introduced v3 of
aws-sdk. This meant that any proxy-related config was done on the
`AWS.CloudwatchLogs` instance, rather than on `WinstonCloudwatch`

Removing this dependency avoids a critical vulnerability with vm2,
inherited via proxy-agent and its dependencies.

Fixes #218
Supersedes #216

Author: LoneRifle