Disable PPC64 tests

I've had enough with Travis. Almost all builds are timing out and
I'm being spammed with "errored e-mails" from Travis. We will not
run the test suite for PPC64 anymore.

.travis.yml

@@ -1,13 +0,0 @@
-env:
-  - TARGET=s390x-unknown-linux-gnu
-arch:
-  - s390x
-os: linux
-dist: focal
-language: rust
-cache: cargo
-rust:
-  - stable
-script:
-  - sudo apt install libhtml-parser-perl
-  - make test