Add heroku-18 to our Travis CI config

- Add stage to Travis CI config and update tests.sh script to recognize
  it

- Update tests to assert there is no Python 2 on Heroku-18

- Update nltk fixture to use Python 3.6 so we can test it on all stacks

Closes gh-730

.travis.yml

@@ -9,6 +9,11 @@ jobs:
         - PATH="/tmp/shellcheck-latest:$PATH"
       script: make check
 
+    - stage: "Stack Tests"
+      services: docker
+      env: STACK=heroku-18
+      script: ./tests.sh
+
     - stage: "Stack Tests"
       services: docker
       env: STACK=heroku-16

Makefile

@@ -18,6 +18,11 @@ test-heroku-16:
 	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=heroku-16" heroku/heroku:16-build bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run;'
 	@echo ""
 
+test-heroku-18:
+	@echo "Running tests in docker (heroku-18)..."
+	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=heroku-18" heroku/heroku:18-build bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run;'
+	@echo ""
+
 buildenv-heroku-16:
 	@echo "Creating build environment (heroku-16)..."
 	@echo

test/fixtures/nltk/runtime.txt

-python-2.7.13
+python-3.6.6
\ No newline at end of file

test/run

@@ -39,6 +39,11 @@ testGEOS() {
 }
 
 testNLTK() {
+  # NOTE: This is a RuntimeWarning emitted by Python 3's runpy.py script
+  # which is what is used when you call `python -m <module>`. This is due to
+  # how nltk imports things. It's not actually an error, but it would probably
+  # be bad to silence in Production.
+  export PYTHONWARNINGS="ignore::RuntimeWarning"
   compile "nltk"
   assertCaptured "Downloading NLTK packages: city_database stopwords"
   assertCapturedSuccess
@@ -76,9 +81,18 @@ testPylibmc() {
 }
 
 testPython2() {
-  compile "python2"
-  assertCaptured "python-2.7.15"
-  assertCapturedSuccess
+  if [[ "$STACK" == "heroku-16" ]] || [[ "$STACK" == "cedar-14" ]]; then
+    compile "python2"
+    assertCaptured "python-2.7.15"
+    assertCapturedSuccess
+  fi
+}
+
+testNoPython2() {
+  if [[ "$STACK" == "heroku-18" ]]; then
+    compile "python2"
+    assertCapturedError
+  fi
 }
 
 testPython3() {

tests.sh

@@ -13,4 +13,9 @@ fi
 if [[ "$STACK" == "heroku-16" ]]; then
     make test-heroku-16
     exit $?
-fi
+fi
\ No newline at end of file
+
+if [[ "$STACK" == "heroku-18" ]]; then
+    make test-heroku-18
+    exit $?
+fi