Adds Semigroup for Validated

[mikejcurry]

Added a Semigroup for Validated based on comment:
https://github.com/non/cats/pull/715#issuecomment-162326224

On a side note, I think there is at least one other Monoid instance (Xor) that has the same characteristics, i.e. no separate Semigroup instance . Seems based on the referenced comment it would be good to add these too?

[ceedubs]

Thanks! Looks good to me. 👍 if/when the build passes.

👍 for your suggestions of an Xor Semigroup as well. Good catch.

[codecov-io]

Current coverage is 85.48%

Merging #716 into master will increase coverage by +0.01% as of 3971b69

@@            master    #716   diff @@
======================================
  Files          162     162       
  Stmts         2237    2239     +2
  Branches        74      74       
  Methods          0       0       
======================================
+ Hit           1912    1914     +2
  Partial          0       0       
  Missed         325     325       

Review entire Coverage Diff as of 3971b69

Powered by Codecov. Updated on successful CI builds.

[mikejcurry]

Pushing a change as per #717 to keep things in line with naming conventions for tests. More specifically, changing the text in the test name from "Monoid[Validated[String, Int]]" to "Validated[String, Int]"

[non]

👍

[non]

Thanks!