Install libgmp in travis

BoeingX · BoeingX · commit abdbc666db30 · 2018-11-18T16:11:43.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -25,10 +25,10 @@ cache:
   - $HOME/.stack
 
 # Ensure necessary system libraries are present
-# addons:
-#   apt:
-#     packages:
-#       - libgmp-dev
+addons:
+  apt:
+    packages:
+      - libgmp-dev
 
 before_install:
 # Download and unpack the stack executable
