multiple_crate_versions lint detects dev dependencies also

[tkaitchuck]

The stated reason for the multiple_crate_versions check is

This bloats the size of targets, and can lead to confusing error messages when structs or traits are used interchangeably between different versions of a crate.

However this error triggers when multiple crates declare a dev dependency on the same target also. For example:

warning: multiple versions for dependency `syn`: 0.15.44, 1.0.11
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#multiple_crate_versions

However because this is a dev dependency it should not cause any of the issues mentioned above. It would be good to exclude dev and test dependencies from this check.