Remove Travis CI for GHC 7.6.x

[BardurArantsson]

The latest GHC 7.6.x was in Apr 2013 and it seems that trying to build
with it on Travis fails because one or more of the dependencies fail
to compile.

[BardurArantsson]

Hm, this looks very much like a spurious failure. The log is just suddenly cut off after a download. Could you please try restarting the CI run?

First commit should be good to go, regardless.

[BardurArantsson]

Oh, I see that there actually was a genuine failure. Last night the Travis log just sort of... stopped.

I'll see if the failure can be fixed tomorrow.

[BardurArantsson]

I'll try splitting everything up into smaller bits and submit piecemeal instead. I'll ping you when I've done all that can be done for now.

[lpsmith]

I think Travis did something to break 7.6. I am not enthusiastic about giving up support for 7.6 though, even if I have to test it myself without Travis's help.

[BardurArantsson]

The problem isn't so much pgsql-simple, it's the aeson dependency - so unless you plan on doing something about that pgsql-simple won't compile on ghc 7.6. On phone right now, I'll explain more fully later.

[BardurArantsson]

Note: Some of the bits that I've removed are actually pre-7.6! I can re-check which bits those are and only remove them if you prefer. However, see below:

This is the end of the log for a recent failed build says:

cabal: Error: some packages failed to install:
aeson-1.2.3.0 failed during the building phase. The exception was: ExitFailure 1
cryptohash-0.11.9 depends on memory-0.14.8 which failed to install.
cryptonite-0.24 depends on memory-0.14.8 which failed to install.
memory-0.14.8 failed during the configure step. The exception was:
user error (The package requires Cabal library version -any && >=1.18 but no suitable version is installed.)

So it's not quite aeson which fails, it just does so indirectly. I do note that memory itself is only complied against >= 7.8.x on Travis CI, so who knows if it'll break again in future even if fixed upstream...

Honestly, given the above I don't think it makes any sense to keep the code around.

[BardurArantsson]

Do let me know what you decide, and I'll rework this PR.