reduce the number of ci builds

Instead of this giant matrix, we're going to focus on a single linux
distro and a few versions of OCaml, and for each one we can use our
`Makefile` rules to build the packages and run tests. It's possible that
I don't completely understand the tradeoffs of doing it this way, but we
should at least have much more reasonable build times.

.travis.yml

@@ -8,17 +8,10 @@ env:
   global:
   - PINS="httpaf-async:. httpaf-lwt:. httpaf:."
   - TESTS=true
-  - POST_INSTALL_HOOK="opam install -t . && opam exec -- make build tests"
+  - POST_INSTALL_HOOK="opam exec -- make build test examples"
+  - PACKAGE="httpaf"
+  - DISTRO="ubuntu"
   matrix:
-  - PACKAGE="httpaf"       DISTRO="ubuntu"          OCAML_VERSION="4.07"
-  - PACKAGE="httpaf-async" DISTRO="alpine"          OCAML_VERSION="4.07"
-  - PACKAGE="httpaf-lwt"   DISTRO="debian-stable"   OCAML_VERSION="4.07"
-  - PACKAGE="httpaf"       DISTRO="ubuntu-lts"      OCAML_VERSION="4.06"
-  - PACKAGE="httpaf-async" DISTRO="fedora"          OCAML_VERSION="4.06"
-  - PACKAGE="httpaf-lwt"   DISTRO="opensuse"        OCAML_VERSION="4.05"
-  - PACKAGE="httpaf"       DISTRO="centos"          OCAML_VERSION="4.04"
-  - PACKAGE="httpaf-async" DISTRO="ubuntu"          OCAML_VERSION="4.04"
-  - PACKAGE="httpaf-lwt"   DISTRO="ubuntu"          OCAML_VERSION="4.04"
-  - PACKAGE="httpaf"       DISTRO="debian-testing"  OCAML_VERSION="4.03"
-  - PACKAGE="httpaf-async" DISTRO="debian-testing"  OCAML_VERSION="4.03"
-  - PACKAGE="httpaf-lwt"   DISTRO="debian-testing"  OCAML_VERSION="4.03"
+  - OCAML_VERSION="4.07"
+  - OCAML_VERSION="4.06"
+  - OCAML_VERSION="4.05"