Update erlang/otp version for ci (#2141)

.travis.yml

@@ -1,7 +1,7 @@
 language: erlang
 
 otp_release:
-   - 21.0.4
+   - 21.2
 
 before_install:
     - git clone https://github.com/erlang/rebar3.git; cd rebar3; ./bootstrap; sudo mv rebar3 /usr/local/bin/; cd ..