Bump upper bounds of package dependencies.

[jonathanknowles]

This PR makes the following adjustments to upper bounds of dependencies:

-   , containers  >=0.4.2.1 && <0.7
+   , containers  >=0.4.2.1 && <0.8

-   , QuickCheck  >=2.14.1  && <2.14.4
+   , QuickCheck  >=2.14.1  && <2.16

Tested with:

cabal test all --constraint=containers==0.7 --constraint=QuickCheck==2.15

Fixes:

• QuickCheck bounds are out of date #92
• Upper bound out of date for containers. #94

[jonathanknowles]

My apologies, I've just noticed:

https://github.com/haskellari/qc-instances/blob/master/CONTRIBUTING.md
If it's really only bounds, please simply open an issue (so you'll have a URL to refer to). I have a semi-automated process to make revisions, pull requests only disturb it.

I'll close this PR, and instead open an issue.

[jonathanknowles]

Related issues:
#92
#94