Cleanup Travis Erlang version testing

Test on one version of 15, 16, 17, and 18

.travis.yml

@@ -1,15 +1,6 @@
 language: erlang
 otp_release:
+  - 18.2.1
   - 17.0
   - R16B03-1
-  - R16B03
-  - R16B02
-  - R16B01
-  - R16B
   - R15B03
-  - R15B02
-  - R15B01
-  - R15B
-  - R14B04
-  - R14B03
-  - R14B02