.travis.yml: sync Erlang and Elixir versions, require Erlang/OTP 21.2

Author: michaelklishin

.travis.yml

@@ -18,7 +18,7 @@ addons:
       # Use Elixir from Erlang Solutions. The provided Elixir is
       # installed with kiex but is old. We also can't use kiex to
       # install a newer one because of GitHub API rate limiting.
-      - elixir=1.6.0-1
+      - elixir=1.7.4-1
 cache:
   apt: true
 env:
@@ -27,8 +27,7 @@ env:
     - secure: CDccdPeAR5bWYcKUzFZwSaUfPmdxBMZwfmArCFmxRntTyZ/ZvZLsVkPUrokLRwMtZSb9sdF2BhoJrfPQqHp8zqxFEYaRQIScBbiSkKHIcC6PVHvLwizQV5PfGnjmHcQ/b/Xdq/XtujYQkoRwYZZvcZ/kBOcRT6xjSBctMrCAF3c=
 
 otp_release:
-  - "19.3"
-  - "20.3"
+  - "21.2.2"
 
 before_script:
   # The checkout made by Travis is a "detached HEAD" and branches
@@ -46,7 +45,7 @@ before_script:
   - |
     echo YES | kiex implode
     elixir --version
-    elixir --version | grep -q 'Elixir 1.6.0'
+    elixir --version | grep -q 'Elixir 1.7.4'
 
 script:
   - make xref