[ travis ] pin down alex & happy versions

Author: gallais

.travis.yml

@@ -12,7 +12,11 @@ cache:
 
 matrix:
   include:
-    - env: TEST=MAIN GHC_VER=8.4.4 BUILD=CABAL CABAL_VER=2.2
+    - env:
+        GHC_VER=8.4.4
+        CABAL_VER=2.2
+        ALEX_VER=3.2.5
+        HAPPY_VER=1.19.12
       addons:
         apt:
           packages:
@@ -22,14 +26,14 @@ matrix:
             - hvr-ghc
 
 before_install:
-  - export PATH=/opt/ghc/$GHC_VER/bin:/opt/cabal/$CABAL_VER/bin:/opt/alex/3.1.7/bin:/opt/happy/1.19.5/bin:~/.cabal/bin/:$PATH;
+  - export PATH=/opt/ghc/$GHC_VER/bin:/opt/cabal/$CABAL_VER/bin:/opt/alex/$ALEX_VER/bin:/opt/happy/$HAPPY_VER/bin:~/.cabal/bin/:$PATH;
 
 install:
   - cabal update
   - sed -i 's/^jobs:/-- jobs:/' $HOME/.cabal/config
 
   # installing dependencies
-  - cabal install alex happy cpphs
+  - cabal install alex-$ALEX_VER happy-$HAPPY_VER cpphs
 
   # installing Agda
   # Even if there seems to be duplicated code for master & experimental,