Replace & update pure unescape config to test the ffi since it's no longer the default

Author: bergmark

.travis.yml

@@ -25,7 +25,7 @@ matrix:
     - env: BUILD=stack CABALVER=1.24 GHCVER=8.0.2 STACK_YAML=stack-bench.yaml
       compiler: ": #GHC 8.0.2"
       addons: {apt: {packages: [cabal-install-1.24,ghc-8.0.2], sources: [hvr-ghc]}}
-    - env: BUILD=stack CABALVER=1.24 GHCVER=8.0.2 STACK_YAML=stack-pure-unescape.yaml
+    - env: BUILD=stack CABALVER=1.24 GHCVER=8.0.2 STACK_YAML=stack-ffi-unescape.yaml
       compiler: ": #GHC 8.0.2"
       addons: {apt: {packages: [cabal-install-1.24,ghc-8.0.2], sources: [hvr-ghc]}}
     - env: BUILD=cabal CABALVER=1.24 GHCVER=8.0.2

stack-ffi-unescape.yaml

@@ -0,0 +1,7 @@
+resolver: nightly-2017-09-04
+packages:
+- '.'
+flags:
+  aeson:
+    fast: true
+    cffi: true