Why does this need a Cabal constraint?

[This build](https://travis-ci.org/ghcjs/jsaddle/jobs/187324575) failed with GHC 7.10 with:

```
+ cabal new-build jsaddle-warp:lib:jsaddle-warp jsaddle-warp:test:test-tool
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: Cabal-1.22.5.0/installed-9ea... (dependency of haskell-gi-0.20)
trying: gi-glib-2.0.11 (dependency of jsaddle-webkit2gtk-0.8.2.0)
trying: ghc-paths-setup.Cabal~>Cabal-1.22.5.0/installed-9ea... (dependency of
ghc-paths-0.1.0.9)
trying: gi-glib-setup.haskell-gi~>haskell-gi-0.20 (dependency of
gi-glib-2.0.11)
next goal: gi-glib-setup.Cabal (dependency of gi-glib-2.0.11)
rejecting:
gi-glib-setup.Cabal~>ghc-paths-setup.Cabal-1.22.5.0/installed-9ea...,
gi-glib-setup.Cabal~>Cabal-1.22.5.0/installed-9ea... (conflict: gi-glib =>
gi-glib-setup.Cabal>=1.24)
rejecting: gi-glib-setup.Cabal-1.22.5.0/installed-9ea... (multiple instances)
rejecting: gi-glib-setup.Cabal-1.24.2.0, gi-glib-setup.Cabal-1.24.0.0,
gi-glib-setup.Cabal-1.22.8.0, gi-glib-setup.Cabal-1.22.7.0,
gi-glib-setup.Cabal-1.22.6.0, gi-glib-setup.Cabal-1.22.5.0,
gi-glib-setup.Cabal-1.22.4.0, gi-glib-setup.Cabal-1.22.3.0,
gi-glib-setup.Cabal-1.22.2.0, gi-glib-setup.Cabal-1.22.1.1,
gi-glib-setup.Cabal-1.22.1.0, gi-glib-setup.Cabal-1.22.0.0,
gi-glib-setup.Cabal-1.20.0.4, gi-glib-setup.Cabal-1.20.0.3,
gi-glib-setup.Cabal-1.20.0.2, gi-glib-setup.Cabal-1.20.0.1,
gi-glib-setup.Cabal-1.20.0.0, gi-glib-setup.Cabal-1.18.1.7,
gi-glib-setup.Cabal-1.18.1.6, gi-glib-setup.Cabal-1.18.1.5,
gi-glib-setup.Cabal-1.18.1.4, gi-glib-setup.Cabal-1.18.1.3,
gi-glib-setup.Cabal-1.18.1.2, gi-glib-setup.Cabal-1.18.1.1,
gi-glib-setup.Cabal-1.18.1, gi-glib-setup.Cabal-1.18.0,
gi-glib-setup.Cabal-1.16.0.3, gi-glib-setup.Cabal-1.16.0.2,
gi-glib-setup.Cabal-1.16.0.1, gi-glib-setup.Cabal-1.16.0,
gi-glib-setup.Cabal-1.14.0, gi-glib-setup.Cabal-1.12.0,
gi-glib-setup.Cabal-1.10.2.0, gi-glib-setup.Cabal-1.10.1.0,
gi-glib-setup.Cabal-1.10.0.0, gi-glib-setup.Cabal-1.8.0.6,
gi-glib-setup.Cabal-1.8.0.4, gi-glib-setup.Cabal-1.8.0.2,
gi-glib-setup.Cabal-1.6.0.3, gi-glib-setup.Cabal-1.6.0.2,
gi-glib-setup.Cabal-1.6.0.1, gi-glib-setup.Cabal-1.4.0.2,
gi-glib-setup.Cabal-1.4.0.1, gi-glib-setup.Cabal-1.4.0.0,
gi-glib-setup.Cabal-1.2.4.0, gi-glib-setup.Cabal-1.2.3.0,
gi-glib-setup.Cabal-1.2.2.0, gi-glib-setup.Cabal-1.2.1,
gi-glib-setup.Cabal-1.1.6, gi-glib-setup.Cabal-1.24.1.0 (dependencies not
linked: cannot make gi-glib-setup.Cabal canonical member of
{*Cabal-1.22.5.0/installed-9ea...,ghc-paths-setup.Cabal-1.22.5.0/installed-9ea...,gi-glib-setup.Cabal-1.22.5.0/installed-9ea...};
conflict set: Cabal, ghc-paths-setup.Cabal, gi-glib-setup.Cabal,
gi-glib-setup.haskell-gi)
Backjump limit reached (currently 2000, change with --max-backjumps or try to
run with --reorder-goals).
```

Adding `--constraint='Cabal>=1.24.2.0'` fixes it.

Adding `--max-backjumps=20000` or `--reorder-goals` causes it to take more than the 10 minute travis timeout for no output.