Travis Builds are broken

[Matthew Pickering]

A recent commit has caused the Travis builds to stop working. I think
it was caused by
https://phabricator.haskell.org/rGHCc2bd62ed62d2fae126819136d428989a7b4ddc79

Example failure

Wrong exit code for plugins01()(expected 0 , actual 2 )
Stderr ( plugins01 ):
ghc-stage2: /home/travis/build/ghc/ghc/libraries/ghci/dist-install/build/HSghci-8.1.o:
unknown symbol `purgeObj'
ghc-stage2: unable to load package `ghci-8.1'
make[2]: *** [plugins01] Error 1
*** unexpected failure for plugins01(normal)

Full log: https://s3.amazonaws.com/archive.travis-ci.org/jobs/190738798/log.txt

Jon, could you perhaps take a look to see if the fix is obvious?

Matt