travis-ci: update otp versions
diff --git a/.travis.yml b/.travis.yml
index 74440e6..edd413f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,7 +12,7 @@
   - 18.0
   - 19.1
   - 20.0.5
-  - 20.1
+  - 20.1.5
 env: MAKE_TARGET=ci
 before_script:
   - hostname -f
@@ -23,7 +23,7 @@
   - find .rebar
 matrix:
   include:
-    - otp_release: 20.1
+    - otp_release: 20.1.5
       env: MAKE_TARGET=ci-dialyze
 script: make $MAKE_TARGET
 cache: