chore: Enable golangci-lint and fix errors (#554)

Enable golangci-lint checking in travis and fix errors identified by it.

This mainly involves removing dead code and improving error checking
such as:
* activeConn.Close now returns any error from operations it performs
  instead of always returning nil.
19 files changed
tree: be496881860b0207ebfa0dd5ba5e583ebc87e7ac
  1. .github/
  2. .travis.yml
  4. README.markdown
  5. go.mod
  6. go.sum
  7. redis/
  8. redisx/