Pin Erlang at 20.3 because the newest from ESL (21) does not work with RMQ

(cherry picked from commit 03542ef616a2a849e8bfb0845427f50e741ea0c6)

.travis.yml

@@ -8,7 +8,33 @@
       - sourceline: deb https://packages.erlang-solutions.com/ubuntu trusty contrib
         key_url: https://packages.erlang-solutions.com/ubuntu/erlang_solutions.asc
     packages:
-      - erlang-nox
+      # apt-cache show erlang-nox=1:20.3-1 | grep Depends | tr ' ' '\n' | grep erlang | grep -v erlang-base-hipe | tr -d ',' | sed 's/$/=1:20.3-1/'
+      - erlang-base=1:20.3-1
+      - erlang-asn1=1:20.3-1
+      - erlang-corba=1:20.3-1
+      - erlang-crypto=1:20.3-1
+      - erlang-diameter=1:20.3-1
+      - erlang-edoc=1:20.3-1
+      - erlang-eldap=1:20.3-1
+      - erlang-erl-docgen=1:20.3-1
+      - erlang-eunit=1:20.3-1
+      - erlang-ic=1:20.3-1
+      - erlang-inets=1:20.3-1
+      - erlang-inviso=1:20.3-1
+      - erlang-mnesia=1:20.3-1
+      - erlang-odbc=1:20.3-1
+      - erlang-os-mon=1:20.3-1
+      - erlang-parsetools=1:20.3-1
+      - erlang-percept=1:20.3-1
+      - erlang-public-key=1:20.3-1
+      - erlang-runtime-tools=1:20.3-1
+      - erlang-snmp=1:20.3-1
+      - erlang-ssh=1:20.3-1
+      - erlang-ssl=1:20.3-1
+      - erlang-syntax-tools=1:20.3-1
+      - erlang-tools=1:20.3-1
+      - erlang-xmerl=1:20.3-1
+      - erlang-nox=1:20.3-1
 
 env:
   global: