Revert "travis: cache local opam switch?"
Revert "travis: cache local opam switch?"

TODO: only create switch if _build does not exist and otherwise just
install into it?
Travis:
$ opam switch -y create . --deps-only ocaml-base-compiler.4.09.0 --locked
[ERROR] Directory "/Users/travis/build/goblint/analyzer/_opam" already exists,
        please choose a different name

This reverts commit ffcd2fcd8c39e62923eb64eb14f56a68de97b468.

goblint/analyzer,goblint/analyzer,goblint/analyzer,goblint/analyzer,goblint/analyzer