Use ubuntu trusty for OTP 18 on Travis

.travis.yml

@@ -9,12 +9,14 @@ env:
   - PATH=".:/usr/lib/postgresql/10/bin:$PATH"
 install: "true"
 language: erlang
-otp_release:
-  - 22.0
-  - 21.3
-  - 20.3
-  - 19.3
-  - 18.3
+matrix:
+  include:
+    - otp_release: 22.0
+    - otp_release: 21.3
+    - otp_release: 20.3
+    - otp_release: 19.3
+    - otp_release: 18.3
+      dist: trusty
 script:
   - make elvis
   - make test