[travis] test only on GHC 7.8

.travis.yml

@@ -1,10 +1,9 @@
 language: haskell
 ghc:
-  - 7.6
   - 7.8
 before_install:
   - "sudo apt-get install libpcre3 libpcre3-dev"
   # register library as a package (needs to be fully installed on the system before the tests are run)
-  - "cabal install --force-reinstalls --enable-tests --max-backjumps 50"
+  - "cabal install --force-reinstalls --enable-tests
 after_script:
   - "cat dist/test/aeson-schema-*-tests.log"