Applied dependency constraints to CI install step

Author: sebright

.travis.yml

@@ -102,6 +102,7 @@ script:
 
  # check that the generated source-distribution can be built & installed
  - export SRC_TGZ=$(cabal-1.18 info . | awk '{print $2 ".tar.gz";exit}') ;
+   cp cabal.config dist;
    cd dist/;
    if [ -f "$SRC_TGZ" ]; then
       cabal-1.18 install "$SRC_TGZ";