elm-format: GHC 8 compatibility
elm-format: GHC 8 compatibility

Without adding "allow-newer" for aeson, base, and transformers, I get
the build failure:

```
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: elm-format-0.3.0 (user goal)
trying: transformers-0.5.2.0/installed-0.5... (dependency of
regex-applicative-0.3.3)
next goal: optparse-applicative (dependency of elm-format-0.3.0)
rejecting: optparse-applicative-0.12.1.0, optparse-applicative-0.12.0.0
(conflict: elm-format => optparse-applicative>=0.11 && <0.12)

etc. etc.
```

Can be removed as soon as elm-format and the dependencies setting unecessary
restrictions on these versions catch up, or as soon as elm-format itself has a
similar built-in workaround.

Closes #46.

Signed-off-by: ilovezfs <fbd54dbbcf9e596abad4ccdc4dfc17f80ebeaee2@icloud.com>

DomT4/homebrew-devel-only