Dependencies don't work.

Bumpity 2

There's probably an environment variable for this...

2.4 has deprecated "cabal install", so previous script failed.

Unsure if this will test fgl-arbitrary as well; will need to try and
see.

Closes #82

Fixing build with MonadFailDesugaring

typo

Use swap from Data.Tuple

Bumped version upper bounds for hspec & QuickCheck

Two conflicting issues:

* cabal-install on Travis-CI was selecting newer versions of
  containers than shipped with the corresponding version of GHC (thus
  hiding the other issue).

* foldlWithKey' was added in containers-0.4.2, and thus not available
  in older versions.  We now fudge it with foldWithKey, which dates
  back to 0.1 (and thus always available).

Duplicate the intent of IM.foldlWithKey' (which was added in 0.4.2)
with foldWithKey.

Closes #69

Currently it brings in the newest version of containers all the time,
which doesn't occur if anything actually depends upon fgl, causing
real world breakage like in #69.

Fix typo in IOArray

Fix one character typo.

Make shortest-path functions safer.

Namely:

- return `Nothing` instead of failing with the dreaded `*** Exception:
  Prelude.head: empty list` (for `spLength`) or returning
  non-sense (for `sp`) when the source is not connected to the
  destination.

- mention in the docs that negative edge weights will not produce the
  right answer.

- add tests that check that `Nothing` and `Just` are returned when
  they should be for shortest-path operations.

Use addLists to concatenate lists instead of (++)