Add OTP 21.0 to build matrix, also try building a package (protocolbuffers#36)

Exclude 1.4 and 1.5 w/ OTP 21.0 since TravisCI doesn't support this (Hex does
not supply such combination yet?)

Author: LOU Xun

.travis.yml

@@ -9,6 +9,13 @@ elixir:
 otp_release:
   - 19.3
   - 20.0
+  - 21.0
+matrix:
+  exclude:
+  - elixir: 1.4
+    otp_release: 21.0
+  - elixir: 1.5
+    otp_release: 21.0
 env:
   - MIX_ENV=test VERBOSE_TESTS=true
 install:
@@ -23,3 +30,4 @@ before_script:
 script:
   - mix test
   - mix test.integration
+  - mix hex.build