Add --enable-shared to cabal flags

Author: yfyf

.travis.yml

@@ -8,7 +8,7 @@ env:
     - secure: "fkytTKJVcJZUW5OTYcKW5D3/hQTwgqfKkguA0nbpqan+1q4amyovAxJLxBPgIn/snyNZV1b7jgwN3TExRUjE0mf0O0tqo9/X5Yg/t+HfpwN2aCDVEmd+7z1jea8GliVNesiamvZfFeaaYzzlNu0zkdf9o0nkHETKY2d/j++jM6Q="
     - CABAL_FLAGS="-j --disable-documentation --disable-tests --reorder-goals
         --disable-library-profiling --disable-library-coverage --enable-library-vanilla
-        --disable-library-for-ghci --disable-benchmarks"
+        --disable-library-for-ghci --disable-benchmarks --enable-shared"
 install:
   - cabal sandbox init
   - cabal install $CABAL_FLAGS -fhttps pandoc