Merge pull request #220 from penhs/master

Update README.md
diff --git a/.travis.yml b/.travis.yml
index 18bf781..ecc72df 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,7 +8,13 @@
   - 21.0
   - 20.0
   - 19.0
-  - 18.3
-  - 17.5
-  - R16B03-1
-  - R15B03
+matrix:
+  include:
+  - otp_release: 18.3
+    dist: trusty
+  - otp_release: 17.5
+    dist: trusty
+  - otp_release: R16B03-1
+    dist: trusty
+  - otp_release: R15B03
+    dist: trusty