Upgrade elixir version to 1.7.4

[kousikmitra]

1. Upgrade elixir version to 1.7.4
2. Update logger_file_backend to 0.0.11

warning: the GenEvent module is deprecated, see its documentation for alternatives
  lib/logger_file_backend.ex:5

warning: GenEvent.__using__/1 is deprecated. Use one of the alternatives described in the documentation for the GenEvent module
  lib/logger_file_backend.ex:5

3. Update retry to 0.11.2
   retry api got changed from 0.10.0, for more details -> https://github.com/safwank/ElixirRetry/tree/v0.11.2

warning: found quoted keyword "credo" but the quotes are not required. Note that keywords are always atoms, even when quoted, and quotes should only be used to introduce keywords with foreign characters in them
  /Users/kousik/Documents/flume/deps/retry/mix.exs:18

warning: found quoted keyword "commit" but the quotes are not required. Note that keywords are always atoms, even when quoted, and quotes should only be used to introduce keywords with foreign characters in them
  /Users/kousik/Documents/flume/deps/retry/mix.exs:20

warning: found quoted keyword "commit" but the quotes are not required. Note that keywords are always atoms, even when quoted, and quotes should only be used to introduce keywords with foreign characters in them
  /Users/kousik/Documents/flume/deps/retry/mix.exs:23

[vasuadari]

Reason we don't change elixir version for a library because we want to support apps which are in lower version. Upgrading elixir would mean that we may have to release a major version. @nitinstp23 what do you think?