xml-conduit: relax transformer upper bounds of cabal constraints

xml-conduit's upper bounds of cabal constraints relaxed as follows:
```log
< 0.4 ->
< 0.5
    ^
```

`resourcet-1.1.2.2` relaxes cabal constraint `transformers < 0.4 → < 0.5.`
`mtl-2.2.x` requires `transformers == 0.4.*`
And transformers-0.4.x have been released.

 So, please relax xml-conduit cabal constraints. It seems to be inconvenience constraints....