language: erlang
otp_release:
   - 22.0
   - 21.3
   - 20.3
   - 19.3
   - 18.3
   - 17.5
   - R16B03-1
   - R15B03

language: erlang
otp_release:
   - 22.0
   - 21.3
   - 20.3
   - 19.3

Remove older Erlang releases (<= R18) from Travis-CI matrix
Remove older Erlang releases (<= R18) from Travis-CI matrix

Looks like they are no longer available for testing.
