test: First try with gitlab-ci

- Add automatic test for `R CMD build`
- Add automatic test for `R CMD check` with `NOT_CRAN` set to `true`

Refs #52

.gitlab-ci.yml

+stages:
+  - build
+  - tests
+
+build:
+  stage: build
+  script:
+    - cd ..
+    - echo "setwd(\"$(pwd)\")" > .Rprofile
+    - R CMD build airgr
+    - mv *.tar.gz airgr/
+  artifacts:
+    untracked: true
+    expire_in: 1 week
+
+check_not_cran:
+  stage: tests
+  script:
+    - echo "setwd(\"$(pwd)\")" > .Rprofile
+    - NOT_CRAN="true"
+    - R CMD check airGR_*.tar.gz