(#5) add travis CI with docker

dependencies are preinstalled in docker image for faster build, but this means we need to update it whenever dependencies change

.travis.yml

@@ -1,2 +1,14 @@
-language: nix
-script: rm -rf .git && nix-build gf.nix
+sudo: required
+
+language: c
+
+services:
+  - docker
+
+before_install:
+  - docker pull odanoburu/gf-src:3.9
+
+script:
+  - |
+    docker run --mount src="$(pwd)",target=/home/gfer,type=bind odanoburu/gf-src /bin/bash -c "cd /home/gfer/src/runtime/c &&
+    autoreconf -i && ./configure && make &&  make install ; cd /home/gfer ; cabal install -fserver -fc-runtime --extra-lib-dirs='/usr/local/lib'"