Make rebar_covertool plugin optional
diff --git a/.travis.yml b/.travis.yml
index 93bbc25..ab8409e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,6 +2,7 @@
 
 os: linux
 otp_release:
+  - 21.0
   - 20.0
   - 19.3
   - 18.3
@@ -16,7 +17,7 @@
   - ./rebar3 dialyzer
   - ./rebar3 edoc
   - ./rebar3 cover
-  - ./rebar3 covertool generate
+  - ./rebar3 as test covertool generate
   - cp _build/test/covertool/passage.covertool.xml ./cobertura.xml
 
 after_success:
diff --git a/rebar.config b/rebar.config
index 60ff529..3118456 100644
--- a/rebar.config
+++ b/rebar.config
@@ -27,13 +27,15 @@
 
 {shell, [{apps, [passage]}]}.
 
-{plugins, [rebar_covertool]}.
-
 {dialyzer,
  [
   {warnings, [error_handling, race_conditions, unmatched_returns, unknown, no_improper_lists]}
  ]}.
 
-{deps,
+{profiles,
  [
+  {test
+   [
+    {plugins, [rebar_covertool]}
+   ]}
  ]}.