Try to keep opam build to avoid recompiling ocaml and Coq each time

.travis.yml

@@ -3,6 +3,9 @@ sudo: required
 language: c
 cache:
   apt: true
+  directories:
+  - $HOME/.opam
+  - $HOME/bin
 
 addons:
   apt:
@@ -21,8 +24,7 @@ env:
 install:
 - curl -sL https://raw.githubusercontent.com/ocaml/opam/master/shell/install.sh > install.sh
 - export PATH=$HOME/bin:$PATH
-- which opam || (rm -rf $HOME/.opam; echo $HOME/bin | sudo sh install.sh --no-backup)
-- opam init --disable-sandboxing -j ${NJOBS} --compiler=${COMPILER} -n -y
+- which opam || (rm -rf $HOME/.opam; echo $HOME/bin | sudo sh install.sh --no-backup; opam init --disable-sandboxing -j ${NJOBS} --compiler=${COMPILER} -n -y)
 - opam --version
 - opam update
 - eval $(opam config env)