travis: disable code coverage

Proptest seems to be hitting this rust-lang/rust#58375

.travis.yml

@@ -1,28 +1,5 @@
 language: rust
 rust:
-    - stable
-    - nightly
-cache: cargo
-# Dependencies of kcov, used by coverage
-addons:
-  apt:
-    packages:
-      - libcurl4-openssl-dev
-      - libelf-dev
-      - libdw-dev
-      - binutils-dev
-      - cmake
-    sources:
-      - kalakris-cmake
-
-sudo: required  # https://github.com/travis-ci/travis-ci/issues/9061
-
-before_script:
-  - cargo install --force cargo-travis
-  - export PATH=$HOME/.cargo/bin:$PATH
-
-script:
-  - cargo test
-
-after_success:
-  - cargo coveralls
+  - stable
+  - beta
+  - nightly