Do we support ghc 7.10?

[DougBurke]

Recent commits (0.2.1.0 and 0.3.0.0) failed to build on ghc 7.10, according to the build matrix on hackage, so I bumped the minimum base version to 4.9 in #24. Then I checked this morning to find that it is back to working on 7.10 (prior to 0.3.0.1 where I bumped the version).

https://matrix.hackage.haskell.org/#/package/hvega/0.3.0.0/ghc-7.10.3@1561428968

So I'll probably revert the change in base, as this is a simple module, and supporting multiple ghc versions isn't too problematic, but is it worth it?

[DougBurke]

Note that the Travis test suite only tests ghc 8.2, 8.4, and 8.6.