fixign issue: Statistics.Quantile.weightedAvg: Sample is empty

- using `cabal sandbox add-source criterion` to
- get the fix from master branch
- see haskell/criterion#161
- should be removed after the new criterion release

.travis.yml

@@ -24,5 +24,5 @@ matrix:
   - ghc: "8.2"
     env: CACHE_NAME=CACHE8.2
 
-install : cabal sandbox init && cabal install --only-dependencies --enable-tests --enable-bench --disable-documentation
+install : cabal sandbox init && git clone https://github.com/bos/criterion.git && cabal add-source criterion && cabal install --only-dependencies --enable-tests --enable-bench --disable-documentation
 script: cabal configure --enable-tests && cabal test --show-details=always && cabal bench