Fix quoted keyword warnings on Elixir 1.7

[ivan]

This fixes:

warning: found quoted keyword "docs" but the quotes are not required. Note that keywords are always atoms, even when quoted, and quotes should only be used to introduced keywords with foreign characters in them
  [...]/deps/cachex/mix.exs:58

warning: found quoted keyword "bench" but the quotes are not required. Note that keywords are always atoms, even when quoted, and quotes should only be used to introduced keywords with foreign characters in them
  [...]/deps/cachex/mix.exs:59

warning: found quoted keyword "cachex" but the quotes are not required. Note that keywords are always atoms, even when quoted, and quotes should only be used to introduced keywords with foreign characters in them
  [...]/deps/cachex/mix.exs:60

warning: found quoted keyword "coveralls" but the quotes are not required. Note that keywords are always atoms, even when quoted, and quotes should only be used to introduced keywords with foreign characters in them
  [...]/deps/cachex/mix.exs:61

warning: found quoted keyword "bench" but the quotes are not required. Note that keywords are always atoms, even when quoted, and quotes should only be used to introduced keywords with foreign characters in them
  [...]/deps/cachex/mix.exs:66

The warning was introduced in elixir-lang/elixir#7838

[whitfin]

It's so gross that Elixir warns you on this, consistency is so much better.

@ivan thanks for the contribution!