Set source encoding so R17 compiler doesn't choke on lager_file_backend

Commit 2b544c40b11ffe24ca41d32267dfd2011c3b747e added a test that assumes the source is latin-1 encoded. R17 defaults everything to utf8 encoding, so trying to compile lager can result in a message similar to:

    deps/lager/src/lager_file_backend.erl:503: cannot parse file, giving up
    ERROR: compile failed while processing deps/lager: rebar_abort

Simply explicitly stating that the source is latin-1 fixes it. I haven't tested on earlier versions of erlang but I assume it's a no-op for anything earlier than R17.