fix dockerfile location typo

Author: CaseyFaist

.travis.yml

@@ -14,7 +14,7 @@ jobs:
     script: make check
   - stage: Build Docker Containers
     name: Build Heroku 18
-    script: docker build --pull --tag heroku-python-build-heroku-18 --file $(pwd)/builds/heroku-18.Dockerfile .
+    script: docker build --pull --tag heroku-python-build-heroku-18 --file $(shell pwd)/Dockerfile.heroku-18 .
   - stage: Test Runs
     script: docker run --rm heroku-python-build-heroku-18 bash test/run-deps
     script: docker run --rm heroku-python-build-heroku-18 bash test/run-versions