Add Travis support for Postgres 9.0, 9.1, 9.2, and 9.3

Previously we just used the default available Postgres;
this ensures pq works correctly on all supported versions.
gofmt
clarify commentary; remove regexp dependency
don't panic
refactor

This changes the behavior slightly: any supported value for
client_encoding is now replaced with the exact string "UTF8".
document client_encoding parameter
Clarify client_encoding error message

We object to the setting, regardless of where it came from, rather
than the env var itself (although the env var handling may need
clearer errors itself--e.g., make the relationship explicit).
Move client_encoding note from godoc to README for consistency

We should consider moving this to the godoc wholesale, but it doesn't
make sense to split the documentation right now.