brew-test-bot: handle changed formulae deps.

If both a formula and its dependencies are changed in a single pull
request it'll fail because the bottle block may no longer be correct.

Handle this case by ignoring bottle pour failures and fetching the
source packages instead of bottles.
cabal-install: bump revision for GCC bump.
cgrep: bump revision for GCC bump.
darcs: bump revision for GCC bump.
git-annex: bump revision for GCC bump.
haskell-platform: bump revision for GCC bump.
pandoc-citeproc: bump revision for GCC bump.
pandoc: bump revision for GCC bump.
shellcheck: bump revision for GCC bump.
cabal-install: use double-quotes.
git-annex: use double-quotes.
haskell-platform: use double-quotes.
cabal-install: update 1.20.0.2_1 bottle.
cgrep: update 6.4.4_1 bottle.
haskell-platform: add 2013.2.0.0_1 bottle.
pandoc: update 1.12.4.2_1 bottle.
pandoc-citeproc: update 0.3.1_1 bottle.
shellcheck: update 0.3.2_1 bottle.